ci: run build on all PRs, not just those targeting master

This allows CI to run on PRs that target branches other than master,
such as feature branches.

Author: c-cube

.github/workflows/main.yml

@@ -4,8 +4,6 @@ on:
     branches:
       - master
   pull_request:
-    branches:
-      - master
 jobs:
   run:
     name: Build