cmd/dist: skip rebuilding packages during builder testing

Since packages in "std" no longer have install targets, checking them
for staleness is somewhat meaningless: if they are not cached they
will be rebuilt anyway, and without installed archives against which
we can compare them the staleness check will not detect builder skew.

It would still be meaningful to check "cmd" for staleness, but
(especially on sharded VM-based builders) that is a fairly expensive
operation relative to its benefit. If we are really interested in
detecting builder skew and/or build reproducibility, we could instead
add a "misc" test (similar to "misc/reboot", or perhaps even a part of
that test) that verifies that bootstrapped binaries are reproducible.

For #57734.
Updates #47257.
Updates #56896.

Change-Id: I8683ee81aefe8fb59cce9484453df9729bdc587c
Reviewed-on: https://go-review.googlesource.com/c/go/+/452775
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Austin Clements <austin@google.com>

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