Give dynamically generated instructions on how to replicate x.py errors

[jyn514]

It can often be difficult to know the minimal command line to reproduce errors, especially when the command CI is running is a full x.py test and the error comes from a test that is rarely run locally. It would be amazing if bootstrap could print how to run the Step for a failed command, to make it easier to replicate the failure locally.

I have some prior work in #86022 that never got merged - a good first step would be to rebase that and address the review comments.

@rustbot label +A-rustbuild +E-hard