CI: consider running fetch-configlet job on a schedule

The fetch-configlet job currently only runs automatically when a fetch-configlet script is changed:

https://github.com/exercism/configlet/blob/26304586b446063ea5f7aa74cd82d29fb4828908/.github/workflows/fetch-configlet.yml#L3-L10

But this means that it could take a while to find out about problems due to e.g. breaking changes from a PowerShell version update (https://github.com/exercism/configlet/issues/839).

We could:

1. Do nothing.
1. Make the job run periodically.
1. Make the job run unconditionally.

This issue is for option 2, which seems reasonable.

Start with .github/workflows/fetch-configlet.yml, especially the trigger configuration at lines 3–10, and inspect how the fetch-configlet job is currently activated. Add a periodic trigger for that job; done means the workflow is configured to run on a schedule as described in the issue, alongside its existing behavior.

Written by the indexing model from the issue text.