misc: use pixi for the source build

[ruben-arts]

Just a test!

If you were interested, this would be fun to merge.

What does it do?

This setup now uses pixi build to build the python source into a conda package, instead of a recipe file.
Running pixi build will actually make a standalone conda package.
Running pixi install will install the package directly into your environment.
This is similar to pypi-dependencies path style. But instead of uv it uses rattler-build to build the package into a installable package. This also works with editable (this is the default).

Changes

• Move pixi specific stuff into the pixi.toml to make the file more readable (not needed but I liked it while working on it)
• Added the [package] table to define the conda metadata in the pixi.toml
• Removed the build environment as that doesn't do much.

TODO

This isn't translated to CI yet. Would first like to know if you would have interest in this.