isos: Ensure proper file ownership and permissions
    
    The genisoimage backend uses the -rational-rock option, which sets uid
    and gid to 0, and makes file readable by everyone.
    
    With xorriso this must be done explicitly. Setting ownership is a single
    command, but the permissions require a per-file command to not make
    files executable where not needed.
    
    Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2203888
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>