cmd/go: place GOROOT/bin at the beginning of PATH in 'go run'
This causes programs that use 'go' as a subprocess to use the same go
command as the parent 'go run' command.
Fixes #68005
Change-Id: I937cef474bf038a925bb74fc73e5f377b03e27b7
GitHub-Last-Rev:
9986537cadb0921763cc1496a63e3806b4aac1ac
GitHub-Pull-Request: golang/go#68040
Reviewed-on: https://go-review.googlesource.com/c/go/+/593255
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>