koji-wrapper: Stop mangling env variables
    
    When koji is authenticated with a keytab, by setting the private
    directory we erased rest of existing environment. In non-keytab path,
    the environment variables got removed as well.
    
    This patch makes sure that the environment will not be modified more
    than necessary (by setting KRB5CCNAME if needed).
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>