Fix copr-build command with results_dir=subdir option
    
    Command copr-build was always assuming to find the srpm in the project
    root directory. However, srpm command is a dependency for copr-build,
    and that command does respect results_dir option. This led copr-build
    to fail when results_dir=subdir was set:
    
        Error: File rust-retry-1.3.1-5.fc40.src.rpm not found
    
    Fix by using srcrpmdir parameter from the project's Layout to find the
    srpm.
    
    Signed-off-by: Otto Liljalaakso <otto.liljalaakso@iki.fi>