codeql: Enable scanning for Actions

[anomiex]

Proposed changes:

GitHub has added code scanning for GitHub Actions workflow files. We may as well turn it on and see what it says.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

None

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• CI happy?

[github-actions]

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[github-advanced-security]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.

[anomiex]

Heh. One of the things it found was "You don't need .github/workflows/codeql-analysis.yml, you can use the default settings." So let's do that.