chore(ci): run backward compat tests on merge to main

- this is to prime cache and check backward data on merge to main

.github/workflows/aws_tfhe_backward_compat_tests.yml

@@ -22,6 +22,9 @@ on:
   # Allows you to run this workflow manually from the Actions tab as an alternative.
   workflow_dispatch:
   pull_request:
+  push:
+    branches:
+      - main
 
 permissions:
   contents: read