don't run impure checks on main

We need to make sure that these tests already pass in pull requests.
There is often no point running this on main because it's already to
late at this point.

.gitea/workflows/checks.yaml

@@ -1,9 +1,6 @@
 name: checks
 on:
   pull_request:
-  push:
-    branches:
-      - main
 jobs:
   checks-impure:
     runs-on: nix