github: enable github actions for postsubmit testing

This gives us a bit of feedback by running our testsuite on Linux,
macOS, and Windows platforms.   While Linux & macOS are passing,
Windows fails some of them.  We can figure that out later.  This
is better than what we have now which is manual one-offs.

Change-Id: I9d2d644be97ec76645db0bc15739e7679310a647
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255314
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>