go: Include all 3 GOROOT tools

Go is supposed to be packaged with 3 tools: cover, vet, and godoc. While most of the things in the go.tools suite can be installed separately, these tools install into $GOROOT instead of $GOPATH. And on NixOS, the $GOROOT directory lives in a read-only subdir in /nix/store.

Include all 3 tools in the standard installation. Right now, godoc tests do not pass, but it seems like that should be tackled as a separate ticket, rather than blocking people's ability to use godoc, vet, and (in particular) cover.

This fixes #3930.