cmd/go: document GOROOT/bin/go PATH entry for go test and go generate

[bcmills]

CL 404134 (in Go 1.19) changed go test and go generate to always place GOROOT/bin at the beginning of the PATH list for the subprocess.

I added a release note for it (in CL 409176), but forget to update the documentation for the actual commands. It should be mentioned explicitly in the documentation for go test and go generate.

[shogohida]

Hi @bcmills, can I work on this? I assume I need to update the documentation in these files.

https://github.com/golang/go/blob/master/src/cmd/go/internal/generate/generate.go
https://github.com/golang/go/blob/master/src/cmd/go/internal/test/test.go

Should I refer to this?

cmd/go: place GOROOT/bin at the beginning of PATH in 'go generate' and 'go test'

This causes tests and generators that execute 'go' as a subprocess to
use the same go command as the parent 'go test' or 'go generate'
command.

[gopherbot]

Change https://go.dev/cl/461683 mentions this issue: cmd/go/internal: update documentation of go test and go generate