feat: allow specifying custom release-plz source for testing

[zvolin]

This allows specifying custom repository url and branch of the release-plz used by the action. If set, instead of fetching the release version with install action, it will clone the specified repository and build the tool locally.
It's likely that you have better ways of testing the release-plz changes that I'm not aware of, but in case not, I found this pretty useful.

example usage
example run

[marcoieni]

Im not too sure this is useful.
Cargo install already has an option to install from git.
So imo if you are already forking the extension you might just use this instead of specifying the options in the action