Add CodeQL workflow

I saw this as part of https://securityscorecards.dev/
which was linked from
https://internals.rust-lang.org/t/security-enhancement-scorecard-github-action

.github/workflows/codeql.yaml

@@ -0,0 +1,36 @@
+
+name: "CodeQL"
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+  schedule:
+    - cron: '30 1 * * 0'
+
+jobs:
+  CodeQL:
+    runs-on: ubuntu-latest
+
+    permissions:
+      # required for all workflows
+      security-events: write
+      actions: read
+      contents: read
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+
+      # Initializes the CodeQL tools for scanning.
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@v2
+        with:
+          languages: cpp
+
+      - name: Build
+        run: ./ci/build.sh
+
+      - name: Perform CodeQL Analysis
+        uses: github/codeql-action/analyze@v2