cmd/dist: run a checkNotStale on the builders before running tests

This should be a no-op, but if the sharded builders are for some
reason distributing stale snapshots — or testing them with mismatched
environments — this should catch them out at a relatively low cost
(#24300 notwithstanding).

Given the frequently at which (*tester).runPending already checks for
staleness, we do not expect the impact of this extra check to be
significant for most builders.

For #33598
Updates #24300

Change-Id: I197d6a69c72e2eec9e4563b459206de76c89e8a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/223755


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>