cmd/dist: fix deadlock when compilation command fails

Can't use bgwait, both because it can only be used from
one goroutine at a time and because it ends up queued
behind all the other pending commands. Use a separate
signaling mechanism so that we can notice we're dying
sooner.

Change-Id: I8652bfa2f9bb5725fa5968d2dd6a745869d01c01
Reviewed-on: https://go-review.googlesource.com/3010


Reviewed-by: Ian Lance Taylor <iant@golang.org>