Update the PR updated_on field when acting on it
Basically this field needs to be manually adjusted when we update a PR,
we do not want to do it automatically using ``onupdate`` so let's do it
manually.
Fixes https://pagure.io/pagure/issue/4159
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>