.gitlab-ci.yml: Ensure we compile before we start the main parallel testing

This build can be as fast as 10mins if the ccache matches and there
are few tests.  Therefore put it first as a sentinal.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
