Add pull request events to CodeQL action

This will validate pull requests from forks to ensure that changes don't end up impacting you negatively.

.github/workflows/codeql.yml

@@ -2,8 +2,12 @@ name: "Code scanning - action"
 
 on:
   push:
+    branches: [master, ]
+  pull_request:
+    # The branches below must be a subset of the branches above
+    branches: [master]
   schedule:
-    - cron: '0 0 * * 0'
+    - cron: '0 15 * * 2'
       
 jobs:
   CodeQL-Build:
@@ -17,6 +21,15 @@ jobs:
     steps:
     - name: Checkout repository
       uses: actions/checkout@v2
+      with:
+        # We must fetch at least the immediate parents so that if this is
+        # a pull request then we can checkout the head.
+        fetch-depth: 2
+
+    # If this run was triggered by a pull request event, then checkout
+    # the head of the pull request instead of the merge commit.
+    - run: git checkout HEAD^2
+      if: ${{ github.event_name == 'pull_request' }}
 
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL