fedora/make-srpm: Do not build SRPM if already available
fedora/make-srpm: Do not build SRPM if already available

A build might be triggered by a change from one of the many repositories
we are polling, only a few of them might contain changes hence
the SRPM previously built can likely be reused.

This will speed up the build process.

hawaii-desktop/builder,hawaii-desktop/builder,hawaii-desktop/builder,hawaii-desktop/builder,hawaii-desktop/builder