actions: run main test CI on a schedule

[oliver-sanders]

Run our main CI actions one a week in order to detect new failures caused by external factors.

This will help us to keep the project status page fresher: https://cylc.github.io/cylc-admin/status/status.html

Examples of external factors:

New lint rules.
New mypy improvements.
Changes in an upstream Cylc repository.
Changes in the [meta-release configuration](https://cylc.github.io/cylc-admin/status/status.html#branches).
New releases of dependent software (incl. Python).
Changes in GH actions (e.g, new runners, or removed support for old ones).
Etc.

[MetRonnie]

IMO too frequent:

• We don't deal with the failures this quickly
• Waste of compute resource

Once a week is probably good enough?

Suggested change

	- cron: '37 04 * * 1-5' # 04:37, Monday-Friday
	- cron: '37 04 * * 0' # 04:37, Sunday

[MetRonnie]

Suggested change

	- cron: '37 04 * */3 1-5' # 04:37 every 3 months
	- cron: '37 04 * */3 *' # 04:37 every 3 months