Repository: go
Issue: cmd/go: respect -cpu=n on workers
Labels: NeedsDecision,fuzz

### What version of Go are you using (`go version`)?

<pre>
λ gotip version
go version devel go1.17-5542c10fbf Fri Jun 4 03:07:33 2021 +0000 windows/amd64
</pre>

### Does this issue reproduce with the latest release?

Tip branch build. Fuzzing not in latest release.

### What operating system and processor architecture are you using (`go env`)?

<details><summary><code>go env</code> Output</summary><br><pre>
λ gotip env                                                                                                                                  
set GO111MODULE=                                                                                                                             
set GOARCH=amd64                                                                                                                             
set GOBIN=                                                                                                                                   
set GOCACHE=d:\temp\gocache                                                                                                                  
set GOENV=C:\Users\klaus\AppData\Roaming\go\env                                                                                              
set GOEXE=.exe                                                                                                                               
set GOFLAGS=                                                                                                                                 
set GOHOSTARCH=amd64                                                                                                                         
set GOHOSTOS=windows                                                                                                                         
set GOINSECURE=                                                                                                                              
set GOMODCACHE=e:\gopath\pkg\mod                                                                                                             
set GONOPROXY=                                                                                                                               
set GONOSUMDB=                                                                                                                               
set GOOS=windows                                                                                                                             
set GOPATH=e:\gopath                                                                                                                         
set GOPRIVATE=                                                                                                                               
set GOPROXY=https://goproxy.io                                                                                                               
set GOROOT=C:\Users\klaus\sdk\gotip                                                                                                          
set GOSUMDB=sum.golang.org                                                                                                                   
set GOTMPDIR=                                                                                                                                
set GOTOOLDIR=C:\Users\klaus\sdk\gotip\pkg\tool\windows_amd64                                                                                
set GOVCS=                                                                                                                                   
set GOVERSION=devel go1.17-5542c10fbf Fri Jun 4 03:07:33 2021 +0000                                                                          
set GCCGO=gccgo                                                                                                                              
set AR=ar                                                                                                                                    
set CC=gcc                                                                                                                                   
set CXX=g++                                                                                                                                  
set CGO_ENABLED=1                                                                                                                            
set GOMOD=NUL                                                                                                                                
set CGO_CFLAGS=-g -O2                                                                                                                        
set CGO_CPPFLAGS=                                                                                                                            
set CGO_CXXFLAGS=-g -O2                                                                                                                      
set CGO_FFLAGS=-g -O2                                                                                                                        
set CGO_LDFLAGS=-g -O2                                                                                                                       
set PKG_CONFIG=pkg-config                                                                                                                    
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=d:\temp\wintemp\go-build102784969=/tmp/go-build -gno-record-gcc-switches 
</pre></details>

### What did you do?

Run [fuzz test](https://gist.github.com/klauspost/1c693c801c87cbdbfbe29db232d31d72) which has a concurrent component, or pretty much any fuzz test.

with `λ gotip test -fuzz=. -test.run=None -parallel=16 -cpu=2`.

### What did you expect to see?

I would expect each worker to have `runtime.GOMAXPROCS(2)` similar to how tests and benchmarks behave.

Arguments could be made for workers defaulting to `runtime.GOMAXPROCS(1)`, but I would at least like the option to change it easily.

### What did you see instead?

Each worker has default `GOMAXPROCS`, leading to significant CPU overprovisioning and workers fighting for CPU threads.