cmd/dist: make test default to --no-rebuild

cmd/dist: make test default to --no-rebuild

I'm tired of having to remember it on every command.
Rebuilding everything is the wrong default.

This CL updates the build script, but the builders may
(or may not) need work, depending on whether they
rebuild using the test command (I doubt it).

Change-Id: I21f202a2f13e73df3f6bd54ae6a317c467b68151
Reviewed-on: https://go-review.googlesource.com/18084
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>