cmd/go: convert TestCacheVet to a script

cmd/go: convert TestCacheVet to a script

This test was failing when GOROOT was read-only.

(I'm not sure why that was the case, but it's simpler to convert to to
a script than to try to debug the non-script test.)

Updates #28387

Change-Id: I9943e28d990e5d8b01da10e70531f3ab99e319a7
Reviewed-on: https://go-review.googlesource.com/c/go/+/206897
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>