ci: fix builds on PRs

Author: drupol

.github/workflows/build-pr.yml

@@ -2,11 +2,13 @@ name: Build PR
 
 on:
     push:
+    pull_request:
 
 jobs:
     build:
         name: Build PDF files
         runs-on: ubuntu-latest
+        if: github.ref != 'refs/heads/main'
 
         steps:
             - name: Set git to use LF