spec: Use RHEL style ovmf-vars-generator bundle
    
    RHEL does not bundle the source archive, but just a copy of the
    script. Use that model and the ovmf-vars-generator syntax RHEL uses.
    
    This part of an effort to make it easier to share edk2 packaging
    across Fedora and RHEL
    
    Signed-off-by: Cole Robinson <crobinso@redhat.com>