Add function to checkout the jobs repository

[ColinKinloch]

Since the gitoxide PrepareFetch and PrepareCheckout structs delete the target path when they are dropped I opted to checkout the repository to a temporary directory, then move it's contents to the path provided to the target directory.

This adds a rather unwieldy GitCheckoutError enum. gitoxides Error types triggers the result_large_err clippy warning.

This requires rust version 1.81 or later.