Replace micromamba with pixi for macos CI

This is just an example to show how we could use pixi instead of micromamba for macOS CI.

The advantage is the pixi.lock that can be easily created for all platforms. Note that if you want to regenerate the lock file from linux, linux-64 has to be defined in the pixi.toml.

Depending on what we want, the pixi.toml file could be simplified. Here I showed how we could test for different cpptango versions (default is for 10.1). If we don't use on Windows, win-64 could be removed.

If you don't want to maintain a pixi.toml in the project, we could eventually keep a "generic" one that would work for most C++ projects in the CI template.