typo fix
    
    Summary:
    Wrong brace used for a str.format() call - caused a crash when
    no universal test image is found for a compose.
    
    Test Plan:
    Check that tests still run. Ideally try testing a compose with
    no universal image (e.g. a Rawhide nightly with no boot.iso).
    
    Reviewers: garretraziel, kparal
    
    Reviewed By: kparal
    
    Subscribers: tflink
    
    Differential Revision: https://phab.qadevel.cloud.fedoraproject.org/D384