goma.mk: do not start goma compiler_proxy if NOSTART_GOMA is set.

During the development of goma client, we sometimes need to use goma
compiler_proxy, which has already been running.

Test: NOSTART_GOMA=1 USE_GOMA=1 m.

Change-Id: Ifa969c40871325fd43ee6443ddbe3b18fde62c5a
1 file changed