ci: Run checks when a commit is pushed to main

This patch causes a CI workflow to trigger whenever a commit is pushed
directly to `main`.  This should fix a broken badge at the top of our
README.

License: GPLv3+
Signed-off-by: Patrick M. Niedzielski <patrick@pniedzielski.net>

Author: pniedzielski

.github/workflows/ci.yaml

@@ -4,6 +4,9 @@ on:
   pull_request:
     branches:
       - main
+  push:
+    branches:
+      - main
 
 concurrency:
   group: ci-${{ github.ref }}-1