Send notifications (email? slack? ...) when daily tests show regressions

[fingolfin]

Sometimes changes on the GAP master branch can cause package tests here to fail. Currently we may not notice if we don't look here. So perhaps we can set up things so that a notification is sent somehow somewhere whenever this happens

[fingolfin]

Alternatively, when a test (run on main, not on a PR or another branch!) detects a failure, it could open an issue for each affected packages, with a link to the failing CI tests, the report, etc. (or, if an automatic issue for that package already exists, update it / comment on it with the new report -- though let's not flood people with two many automatic updates).

Ideally, when regression goes away, it'd also close the issue automatically, but that's less important.