test: prevent running both push and pull_request actions at the same time (#3597)

Not a 100% perfect solution, however as us maintainers always push branches directly to the original repository and make PRs from those, this is plenty enough. Instead of having 300+ actions in each PR which takes time, we'll just have the normal 171 `push` actions.

This makes sure to still run `pull_request` actions from contributors as they'd be forking the repo.

Signed-off-by: Sami Mazouz <ilyasmazouz@gmail.com>

.github/workflows/REUSABLE_backend.yml

@@ -88,7 +88,9 @@ jobs:
 
     name: 'PHP ${{ matrix.php }} / ${{ matrix.db }} ${{ matrix.prefixStr }}'
 
-    if: inputs.enable_backend_testing
+    if: >-
+      inputs.enable_backend_testing &&
+      ((github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository) || github.event_name != 'pull_request')
 
     steps:
       - uses: actions/checkout@master

.github/workflows/REUSABLE_frontend.yml

@@ -91,6 +91,9 @@ jobs:
     name: Checks & Build
     runs-on: ubuntu-latest
 
+    if: >-
+      ((github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name != github.repository) || github.event_name != 'pull_request')
+
     steps:
       - name: Check out code
         uses: actions/checkout@v2