ci: Run CI for stacked PRs (#12)

I'll use stacked PRs in order to make reviewing possible while still not
getting blocked by waiting for an approval from a reviewer. To get this
working the CI must run on PRs to other branches in addition to main

Author: tzanko-matev

.github/workflows/ci.yml

@@ -4,7 +4,8 @@ on:
   push:
     branches: [main]
   pull_request:
-    branches: [main]
+    branches: ['**']
+    types: [opened, synchronize, reopened, ready_for_review]
 
 jobs:
   nix-tests: