avoid running tests twice for pull requests

This implements the suggested method from [1]. It runs tests on all pushes to
our own repository and for pull requests coming from other people's forks.

[1] https://github.community/t/how-to-trigger-an-action-on-push-or-pull-request-but-not-both/16662/10

.github/workflows/phpCS.yml

@@ -1,6 +1,10 @@
 name: PHP Code Style
 
-on: [push, pull_request]
+on:
+    push:
+    pull_request:
+        branches:
+        - '**:**'
 
 jobs:
     phpcs:

.github/workflows/testLinux.yml

@@ -1,6 +1,10 @@
 name: Linux Unit Tests
 
-on: [push, pull_request]
+on:
+    push:
+    pull_request:
+        branches:
+            - '**:**'
 
 jobs:
     run:

.github/workflows/testWindows.yml

@@ -1,6 +1,10 @@
 name: Windows Unit Tests
 
-on: [push, pull_request]
+on:
+    push:
+    pull_request:
+        branches:
+            - '**:**'
 
 jobs:
     run: