SEC: enable security scan for github actions using zizmor

[neutrinoceros]

I bumped the erfa submodule to include liberfa/erfa#104

[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.

[neutrinoceros]

While I was at it, I included the missing comment requested by @mhvk in #183

[neutrinoceros]

rebased to resolve conflicts

[mhvk]

Got the indirect ping via the rebase. I think this all makes sense (though I don't exactly understand what the scan actually entails...). Anyway, since we're doing it for erfa, it makes sense to do it here too.