Function run_prepare_command 

pub fn run_prepare_command(
    service: &str,
    repo: &str,
    reference: &str,
    command: &str,
    env: &[(String, String)],
) -> Result<(), PrepareFailure>

Shallow-clone repo@reference into a temp dir, run command there with env, and clean up. Any failure is returned as a PrepareFailure.