Fix a bug in the logic to skip building pull requests from the same repository

.github/workflows/build.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build:
     # Skip building pull requests from the same repository
-    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != '${{ github.repository }}'
+    if: ${{ github.event_name == 'push' || (github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository) }}
     runs-on: ${{ matrix.os }}
     strategy:
       fail-fast: false