ci: fix double workflow trigger on PR

.github/workflows/build.yml

@@ -2,6 +2,9 @@ name: "Build and populate cache"
 on:
   pull_request:
   push:
+    branches:
+      - main
+      - master
   schedule:
     # rebuild everyday at 2:51
     # TIP: Choose a random time here so not all repositories are build at once: