cmd/go: unset CC when we remove it from PATH in TestScript/cgo_stale_precompiled
Otherwise, if make.bash produced a relative default CC path but the
user has an absolute path to CC set in their environment, the test
will fail spuriously.
For #64423.
Change-Id: I0f3e1d04851585e1b39266badcda9f17489332d9
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/547997
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>