CI images: don't build on a schedule for forks

It's a bit annoying having to *manually* disable a workflow for forks,
because the scheduled build is a waste of time and also spammy.

On the other hand, having the workflow enabled is in general a good
thing, because it is in fact desirable to test it locally in forks.

.github/workflows/images.yml

@@ -25,6 +25,8 @@ on:
 
 jobs:
   build:
+    # do not run the weekly scheduled job in a fork
+    if: github.event_name != 'schedule' || github.repository == 'mesonbuild/meson'
     env:
       HAS_DOCKER_CREDENTIALS: ${{ secrets.DOCKER_PASSWORD != '' }}
     name: ${{ matrix.cfg.name }}