buildinstall: Copy files in thread
    
    Instead of running the copy from the main script explicitly, make it
    part of the thread.
    
    This should make things very slightly faster, and the code is much
    simpler.
    
    Fixes: https://pagure.io/pungi/issue/959
    JIRA: COMPOSE-2604
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>