cloud-image-uploader: use .cert suffix to appease skopeo
    
    Skopeo fails to login with:
    
    FATA[0000] authenticating creds for "registry.stg.fedoraproject.org": creating new docker client:
    missing client certificate registry-fedoraproject.cert for key registry-fedoraproject.key
    
    It seems it is rather picky about the suffix, even though the docs
    mentioned both crt and cert being used.