👷 github: workflows: reuse: Avoid running twice on PRs

It should run only on any PRs against main, and on pushes to main itself
now.

Suggested-by: Stefan Haun <[email protected]>
Link: #26 (comment)
Link: https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/triggering-a-workflow

Author: LeSpocky

.github/workflows/reuse.yml

@@ -3,7 +3,13 @@
 ---
 name: REUSE Compliance Check
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
 
 permissions:
   contents: read