WIP: Introduce an one-off-build command to reproduce ofBorg locally

[andir]

Motivated by trying to review a change in how ofBorg reports build errors I started hacking on a command that allowed me to execute just the build part of ofBorg without setting up a local queue or config files.

This is still a WIP that is hardcoded to exactly one PR and commit revision. I plan to convert all the constants to Clap CLI arguments once I moved the build system of the project to naersk and can then introduce new dependencies.

Things to be done:

• [ ] Move constants to Clap
  • [ ] The system attribute should be defaulted to the current system. We can use the target_os and target_arch values for that. The user should still be able to override thos tho...
• [ ] Make the logging a bit prettier
• [ ] Implement a custom Notification receiver that just logs the messages as they come in.
• [ ] (idea) add support for the eval part as well so we can run ofBorg locally just like we can run nixpkgs-review.