Set Integration CI to run weekly for all repos

[ca-scribner]

As per decisions from the July 13 tech talk, we should run the integration tests for all our charms weekly (in addition to on PRs/pushes). This is to catch things like out-of-date libraries or build/dependency problems when they happen, rather than when someone revisits a charm to do something unrelated.

These tests should:

• run overnight before the Thursday Ticket Guru meeting
• include both functional tests (eg: unit/integration) and checks for out-of-date libraries

This was raised in part because observability libs were updated to fix a bug, but some of our charms went a few months without this fix because we were not otherwise editing the charm.