Refactor cache-pixi-lock.yml workflow into external action

[VeckoTheGecko]

• Followup on Migrate development workflow to Pixi #10888 (comment)

Refactors the workflow to https://github.com/Parcels-code/pixi-lock - outside the Xarray repo so that it can be used in
other projects.

Prefix devs expressed interest in their Discord about maintaining the action, but that's
undecided at the moment
(context).
Either way, I don't think its a blocker (once the repo is transfered, either to prefix-dev or xarray-contrib, the existing links will still work via GHs repo moving symlink behaviour )

Updated the runner for the caching to use ubuntu-slim as they start quicker, and most hits will just be restoring the cache file anyway.

cc @keewis

[keewis]

took me a while to review, but I agree that we don't need to wait on this to be moved outside the parcels org.

I've gone ahead and changed the job dependency chains (there's no need to resolve environments if we skip ci, anyways), and also simplified some trigger conditions. As such I won't approve (as in, the github review reaction), but as far as I can see this looks good to me.