cmd/go: make env -w and -u validate GOOS and GOARCH values
This change makes go env -w and -u check invalid GOOS and GOARCH values and abort if that's the case.
Fixes #34194
Change-Id: Idca8e93bb0b190fd273bf786c925be7993c24a2b
GitHub-Last-Rev:
ee67f09d75f4552001cb8b6506bc4af0894c9b05
GitHub-Pull-Request: golang/go#34221
Reviewed-on: https://go-review.googlesource.com/c/go/+/194617
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>