misc/dashboard/builder: always do a native build

https://codereview.appspot.com/8134043 disabled cgo when cross compiling, this means builders which compile for both amd64 and 386 will be compiling the latter with cgo disabled.

This proposal modifies the builder to mirror the dist tool by always doing a native build.

Tested on my darwin/amd64 builder and confirmed the result when building darwin/386 is a native 386 build with cgo enabled.

R=bradfitz, dsymonds, r, adg
CC=golang-dev
https://codereview.appspot.com/8842044