check notification failure after validation

[xmo-odoo]

If a PR is approved, and CI fails, it's supposed to be notified since 2019 at least. But apparently that doesn't work correctly: odoo/o-spreadsheet#4491

• created at 14:44
• approved at 14:45
• ci/runbot failed at 14:51 (failure still visible in previous_failure field)

Then nothing happens until the PR is updated at 08:24 the day after