Remove unneeded CI token, enable CodeQL, and add security policy

[iamjoemccormick]

Makes the same change as were done for Go. Note unfortunately CodeQL does not yet support Rust, so this only adds scanning for any issues with GitHub Actions workflows.

[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.

[rustybee42]

Look good!

[rustybee42]

I just noticed that now that git checkout replacement config is gone, setting CARGO_NET_GIT_FETCH_WITH_CLI is most likely also not required anymore.

I'll leave it to you though.

[iamjoemccormick]

I just noticed that now that git checkout replacement config is gone, setting CARGO_NET_GIT_FETCH_WITH_CLI is most likely also not required anymore.

I'll leave it to you though.

I left it for now as from what I can tell in the docs there are no drawbacks outside perhaps a slight performance impact of forking a new git process instead of using the mechanisms built in to cargo (which may have other optimizations).