cmd/go/internal/cache: disable builds if GOCACHE is not an absolute path

If GOCACHE is set but is not an absolute path, we cannot build.
And GOCACHE=off also returns the error message "build cache is
disabled by GOCACHE=off".

Fixes #30447

Change-Id: I24f64bc886599ca0acd757acada4714aebe4d3ae
Reviewed-on: https://go-review.googlesource.com/c/164200


Run-TryBot: Baokun Lee <nototon@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>