Build/Test Tools: Address some issues in GitHub Actions workflow files as reported by Zizmor.

This removes unnecessarily broad inheritance of secrets, replaces some GitHub Actions expressions with environment variables, removes git credential persistence, and adds documentation to the readme.

See #64227


git-svn-id: https://develop.svn.wordpress.org/trunk@62251 602fd350-edb4-49c9-b593-d223f7449a82