Run GitHub pull request checks on push to develop

This will make sure tests still pass on the merged code base. Also, this
will make sure caches are actually available on first run in pull
requests: GitHub caches are access restricted and caches created by
branches in forks will not be considered. Only cache entries for the
base branch are always available.

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -1,5 +1,7 @@
 name: Build and Test CBMC
 on:
+  push:
+    branches: [ develop ]
   pull_request:
     branches: [ develop ]
 env: