ci: only run big checks on PRs to main

Signed-off-by: Christina Sørensen <ces@fem.gg>

.github/workflows/unit-tests.yml

@@ -3,7 +3,9 @@
 name: Unit tests
 on:
   push:
+    branches: [main]
   pull_request:
+    branches: [main]
   workflow_dispatch:
 
 concurrency: