CI: keep the cache alive in cron

[bsipocz]

We don't want to use the cache in cron and hacking some extra logic into tox felt an overkill, so I'm just adding a simple job that does the deed.

[bsipocz]

I double checked with a temp commit that this logic works without adding a code checkout step (https://github.com/Caltech-IPAC/irsa-tutorials/actions/runs/36570659692/job/109413542343?pr=387). So will just go ahead and merge this now. My expectation is that this will be enough to keep the cache alive, and as a secondary line of defence we can manually trigger the job, too, in which the build job will use the cache and thus runs fast.

In cron we keep the logic of not using the cache as a way to test for any breakage used by upstream and/or server side changes.