Pin GitHub Actions and narrow permissions

[anoadragon453]

Harden our GitHub Actions via the zizmor GitHub Actions security static analysis tool.

zizmor --fix=all .github now passes.

This PR also adds zizmor as an action to this repository, so we don't regress the current config.

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.