ci: Downgrade to Ubuntu 18.04 for CodeQL env

It emits a setup warning because of a missing Python 2 pip otherwise:
https://github.com/github/codeql-action/pull/316

.github/workflows/ci.yml

@@ -164,7 +164,7 @@ jobs:
   codeql:
     if: "!contains(github.event.head_commit.message, '[ci skip]')"
     timeout-minutes: 30
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-18.04
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2