cmd/dist: run vet/all on dedicated builders

cmd/dist: run vet/all on dedicated builders

We will soon add dedicated builders for running vet/all.
Their name will end with "-vetall".
On those builders, run vet/all and nothing else.
On all other builders, including local all.bash,
don't run vet/all at all, because it is slow.

This will probably be refined more over time.

Change-Id: Ib1d0337adda84353931a325515c132068d4320cd
Reviewed-on: https://go-review.googlesource.com/28962
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>