support pixi environments

[keewis]

• closes support pixi #30
• also closes better error messages for policy schema failures #34 and closes error msg if version is missing #35

This adds initial support for pixi environments.

It does so by manually parsing the manifest file (which needs to be passed explicitly to the action), extracting the features from the given environment (the name of the env is passed as pixi:<env-name>) and merging them into a single list of pins.

There's still a lot to be done before it can be merged:

• PyPI dependencies are ignored without warning warns now
• the tests are lacking / non-existent
• no documentation

cc @VeckoTheGecko