CI: run GCC unit test job on push to main

Allows to get coverage data on coveralls.io

.github/workflows/unit_tests.yml

@@ -3,6 +3,10 @@
 #
 name: Unit tests
 on:
+  # On push/merge to main we only run the GCC job, to get coverage data uploaded to coveralls.io
+  push:
+    branches:
+      - main
   pull_request:
     branches:
       - main
@@ -17,10 +21,13 @@ jobs:
     steps:
       - name: Repository checkout
         uses: actions/checkout@v2
+        if: github.event_name == 'pull_request' || matrix.run_phase == 'GCC'
       - name: Install build dependencies
         run: sudo -E .github/workflows/unit_tests.sh SETUP
+        if: github.event_name == 'pull_request' || matrix.run_phase == 'GCC'
       - name: Build & test (${{ matrix.run_phase }})
         run: sudo -E .github/workflows/unit_tests.sh RUN_${{ matrix.run_phase }}
+        if: github.event_name == 'pull_request' || matrix.run_phase == 'GCC'
       - name: Coveralls
         if: matrix.run_phase == 'GCC' && github.repository == 'systemd/systemd'
         uses: coverallsapp/github-action@master