cmd/dist: disable test caching during run.bash

Sometimes people use run.bash repeatedly
or run go tool dist test by hand for cgo tests.
Avoid test caching in that case, by request.

Refactor code so that all go test commands
share a common prefix.

If not caching is problematic it will be a one-line
change to turn caching back on.

Fixes #22758.

Change-Id: I17d721b832d97bffe26629d21f85b05dbbf2b3ec
Reviewed-on: https://go-review.googlesource.com/80735
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>