Add a scheduled workflow to automatically update the pixi.lock file

[diegoferigo]

It would be nice to have a scheduled workflow (let's say, once per month) that re-creates from scratch a new pixi.lock file and run the tests on the new dependencies. If successful, a new PR can be automatically opened so that the new file can be pushed to LFS.

This would allow to maintain a rolling-release set of working dependencies attached to the git repository.

Related to #189.

[flferretti]

A possible idea could be to modify https://pixi.sh/dev/advanced/updates_github_actions/#how-to-use integrating LFS and logging some diff in the PR description.

[diegoferigo]

Yep, we already discussed this in #189 (comment).