cmd/go: enable fips test and fix caching bug

Enable the cmd/go fips test now that v1.0.0.zip has been checked in.
Will still need to enable the alias half when the alias is checked in.

Also fix a problem that was causing spurious failures, by fixing
repeated unpackings and also disabling modindex reads of the
virtual fips140 snapshot directories.

Fixes #71491.

Change-Id: I7fa21e9bde07ff4eb6c3483e99d49316ee0ea7f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/645835


Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>