Add a timed weekly/monthly (?) build?

[drvinceknight]

With GitHub actions it's relatively simply to run a timed build of the CI. This would potentially alert us to bugs that appears as other libraries update. Here is an example of a weekly build on one of
my class repos: https://github.com/drvinceknight/cfm/blob/master/.github/workflows/config.yml#L7

I don't necessarily think we need this but it's a relatively simple thing to implement. Any thoughts?

[meatballs]

If it's simple to do, why not? It gives a bit of warning that something has gone wrong.

[drvinceknight]

What do you reckon, monthly? Weekly?

[meatballs]

Weekly seems ok to me.

[drvinceknight]

Cool, I'll throw that in a PR.