cmd/go: set FOSSIL_HOME in TestScript/mod_get_fossil

Without HOME or FOSSIL_HOME set, this test fails for me when run with
fossil 2.12.1.

Also verify that the 'go get' command produces an executable, which
helps to verify that the files extracted by fossil are not corrupted.

Updates #42323

Change-Id: Ie6f5d2eab6a6338e997a4f85591195e5bd9a0d37
Reviewed-on: https://go-review.googlesource.com/c/go/+/267884


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>