backend: don't run appstream-builder for PR dirs
    
    The ':pr:' directories are created dynamically, so we can not
    request admins to touch '.disable-appstream' file for every such
    dir.  We would have to check '../../<main_dir>' for that file.
    
    But first it would be slightly more complicated, and second it
    doesn't make much sense to pay attention to it because the ':pr:'
    directories are usually much smaller than the main directory (so
    appstream-builder is cheap operation).
    
    This patch is triggered by the same AssertionError as previous
    commit.