spec: strengthen altfiles sed invocation
    
    The regex that was given matched the `group` line but not the `passwd`
    line (which doesn't have that `[SUCCESS=merge]` bit).
    
    Generalize the regex instead so that we just take whatever the contents
    of the conditional was and put it in there.
    
    Fixes: https://github.com/coreos/fedora-coreos-tracker/issues/1680
    Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2266344