cmd/go: reject relative paths in GOMODCACHE environment
Go already rejects relative paths in a couple environment variables,
It should reject relative paths in GOMODCACHE.
Fixes #43715
Change-Id: Id1ceff839c7ab21c00cf4ace45ce48324733a526
Reviewed-on: https://go-review.googlesource.com/c/go/+/284432
Run-TryBot: Baokun Lee <bk@golangcn.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Jay Conrod <jayconrod@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Trust: Baokun Lee <bk@golangcn.org>