github: fix CodeQL token permissions (#6676)

.github/workflows/codeql.yml

@@ -20,6 +20,10 @@ on:
   schedule:
     - cron: '0 19 * * 0'
 
+permissions:
+  contents: read
+  security-events: write
+
 jobs:
   analyze:
     name: Analyze
@@ -41,11 +45,6 @@ jobs:
           # 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
         uses: github/codeql-action/init@v1