Let user customize conda root, and check env vars.

[claytharrison]

Following up from #32 (comment)_

Currently it's required for the prefix field to be set in a file's environment.yml in order for pet-virtualenv-root to find the right environment. I don't have this in any of my environment.ymls though, and it's keeping emacs-pet from finding the place where my environments live.
https://github.com/wyuenho/emacs-pet/blob/cc086d91a9e6ab0158335d37a774356414746e17/pet.el#L605C1-L609C155

I could of course add a prefix line, and doing so fixes the issue, but if I'm working on a library that's going to be used by anybody else, that line shouldn't be getting checked into the repo. It would be nice if it were possible to point emacs-pet to the micromamba (or conda, mamba - I assume) root prefix in some other way.

This PR adds a custom variable pet-condaish-root that the user can set to point to whatever folder contains their envs/, and pet-condaish-env-path to check that as well as the environment variables CONDA_ROOT and MAMBA_ROOT_PREFIX before pet-virtualenv-root errors out.

The prefix line in the environment.yml gets priority right now, then pet-condaish-root, and then the environmental variables, though I'm not sure if that's the best order or not.

[wyuenho]

Thanks for the report. I've pushed dc176e5 which doesn't require customization. Let me know if that works for you.

[claytharrison]

Yes it works great, thank you!