Fix (hackily) install of X, Firefox and haveged on Modular
    
    Modular composes don't include these packages, but we need them
    to run the web UI tests for FreeIPA and Cockpit. This is the
    most reasonable hack I can come up with for now: just use a
    non-modular fedora repo to source these packages when doing
    Modular compose testing.
    
    If we ever reach an all-Modular future, these packages should
    be available in Modular composes I guess, but for now they are
    not.