workflow: Only run once on PR or merge to main

Restrict when the workflow is run so that it will either run when pushed
to main, or when a pull request to main, and not for arbitrary other
branches.

Signed-off-by: David Brown <[email protected]>

Author: d3zd3z

.github/workflows/build.yml

@@ -7,7 +7,11 @@ name: Build
 
 on:
   push:
+    branches:
+      - main
   pull_request:
+    branches:
+      - main
   schedule:
     - cron: "0 0 * * *"