cmd/go: don't stamp build or vcs info for GOROOT binaries

Fixes a test failure in cmd/go TestScript/mod_outside.

make.bash (cmd/dist) builds everything with -gcflags=all= -ldflags=all=
by default. If those no-op flags aren't used, all GOROOT binaries
appear stale.

It's likely safe to omit those flags in cmd/dist if they're
empty. Checking out a new commit in GOROOT would always cause
staleness since the VCS info would change.

For #37475

Change-Id: Ic9aa0f3b7318e05fbb2f7d2c008ad07a4c61952f
Reviewed-on: https://go-review.googlesource.com/c/go/+/356014


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>