Feature request: configurable MANIFEST_URL and downloadUrl for mirrors

Hi team 👋

Description:
I’d like to ask if setup-python could support customizing MANIFEST_URL and downloadUrl.
In self-hosted environments like Gitea, I mirror GitHub Action repositories locally due to firewall restrictions.
Currently, some URLs inside the action are hardcoded to GitHub (e.g., MANIFEST_URL), which prevents using mirrors.
It would be helpful to allow overriding these URLs via with: parameters so mirrored sources can be used.

Justification:

• I maintain a self-hosted Gitea instance and mirror actions to avoid firewall issues.
• Overriding hardcoded URLs would allow users in restricted regions to use mirrors without modifying the action code.
• Manual code modification is inconvenient because I still periodically sync updates from upstream. Supporting this feature would simplify maintenance and increase usability for self-hosted deployments.

Are you willing to submit a PR?
No, I am just proposing the feature.

Thanks so much for your work on setup-python! I really appreciate your time and consideration. 🙏