Update CI and pixi with test feature

[claudiushaag]

This should be what we talked about.

the task pytest is now unique, so calling it always invokes the test env.
I changed all ocurreces to use the test environment, except for the new CI jobs.
That should make it ready for pixi-build.

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.

[sanathkeshav]

Look good to me!