[Feature] Pin Pixi version on CI

[claudiushaag]

This should maybe minimize errors like in #124 where the pixi on CI demands a relock, presumably because of a higher pixi-version on CI than locally. Could also/additionally be some pixi-build bugs... Nevertheless, a pin might be a good idea.

Pixi is backward compatible with the lock file, but not forward compatible. This means that you can use an older lock file with a newer version of pixi, but not the other way around.

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[claudiushaag]

A lock was needed, will probably need a rebase before merging after #123 or so...