Remove scheduled pixi auto-update and add scheduled build with latest dependencies

[traversaro]

This is a bit of iteration on #178 and what is described in prefix-dev/pixi#910 .

The idea of #178 is that once in a while we wanted to update the pixi lock file, to ensure that the software does not stop working with new versions of the dependencies. However, periodic update of the pixi lock file have the downside that the repo size continue to grow, even if the repo is not changed at all. Furthermore, the PR for the update for the lock needs to be manually merged, and they add noise in the PRs for the repo.

After a bit of experience with the solution added in #178, I thought of another solution: added a scheduled job that deletes the pixi.lock and run the CI on a freshly created pixi.lock, to test the compilation and tests with the latest version of the dependencies (as we used to do with the conda-ci). In this way, we frequently test the latest versions, without changing the pixi.lock, unless necessary. Note that I left the pixi-auto-update-ci action, as it can be useful to update the pixi.lock via workflow_dispatch via the GitHub Actions. The frequency of the scheduled job is bi-weekly.