Test for socket being writable, not just existing
The current system really only works if building the RPM as root
(ewww) or if your user is in the `docker` group, which we discourage
in favor of sudo. I'm not sure how to plumb through sudo, but anyways
it's more accurate to test writability.
Signed-off-by: Lokesh Mandvekar <firstname.lastname@example.org>