cmd/dist: redirect acid output to file to separate from errors

If runtime's proc.c does not compile, cmd/dist used to show
the compile errors in a sea of acid output, making them impossible
to find. Change the command invocation to write the acid output
to a file, so that the errors are the only thing shown on failure.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7221082