Enable Github Actions also for Pull Requests

Author: vogelsgesang

.github/workflows/build.yml

@@ -3,7 +3,7 @@ name: Build
 on:
   # For the time being, only build binaries for releases.
   # Maybe we also want to build for pull requests as soon as this is open-sourced.
-  # pull_request:
+  pull_request:
   push:
   workflow_dispatch:
   release: