Add option to use existing Python installation

[psalz]

Due to this unresolved issue with the setup-python action I'm currently unable to use this action on our self-hosted runners. Since we already have Python 3.12 pre-installed on the runners however, it would be great if there was an option to skip the installation step and simply use the existing installation instead!

Edit: Or maybe even better a python-path option, that if set, skips the setup-python action and uses it to create the venv (i.e. something like "${{ steps.setup-python.outputs.python-path || inputs.python_path }}" -m venv "${GITHUB_ACTION_PATH}/venv")

[platisd]

I can take a look but don't have a good way of testing it. Sounds like you have a good solution to the issue though?
Feel free to provide a PR that addresses this in the meanwhile.