codeql-analysis: Ignore CodeQL analysis in private repositories.

This is not a frequent problem, but causes CI to fail since CodeQL only
runs in public repositories.

Author: neiljp

.github/workflows/codeql-analysis.yml

@@ -20,6 +20,7 @@ permissions:
 jobs:
   analyse:
     name: Analyse
+    if: ${{!github.event.repository.private}}
     permissions:
       actions: read
       contents: read