[CI] Add workflow update-devenv

[straight-shoota]

Example PR: #16386

[ysbaddaden]

What's the point of devenv.lock if we keep updating it every week automatically?

[straight-shoota]

It describes the current state and ensures consistency.
But since we're following a rolling release of nixpkgs, we can get regular updates.
I don't think there's any reason to prevent that?
We can talk about the interval, of course.

[ysbaddaden]

What's the benefit of following the rolling release of nixpkgs instead of the latest stable?

[ysbaddaden]

I mean, it sounds like mere noise and hassle to keep the environment updated. We could instead follow nixpkgs releases, that updates twice a year for example.

[straight-shoota]

Then we'd be using older versions of the tools. I don't think this would be a good idea.

[straight-shoota]

I'd rather have the validation hooks up-to-date than risk falling out of sync with the versions we use and miss critical validation.
Devenv itself is also actively developed and has relatively frequent new releases.

I'm happy to try anything else, though. Just felt it's a nuisance to do this maintenance job manually.

[ysbaddaden]

A 6 months span doesn't feel like we'd lag far behind to me bu 🤷

[straight-shoota]

I've been waiting on an update for actionlint for a couple of weeks already. 😅
It doesn't recognize the new GitHub action runner value macos-15-intel yet, so pre-commit currently fails on macos.yml workflow: https://github.com/crystal-lang/crystal/actions/runs/19548833173/job/55974600067