cmd/go: tests: rename git-min-vers->git-sha256

cmd/go: tests: rename git-min-vers->git-sha256

Follow-up cleanup on go.dev/cl/698835. Clarify the tests by replacing
git-min-vers with a more-puposeful git-sha256 verb in the cmd tests.

Updates: #73704

Change-Id: I4361356431d567a6a3bb3a50eadfaa9e3ba37d90
Reviewed-on: https://go-review.googlesource.com/c/go/+/720481
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>