[ENH] - Allow any env vars to be passed through environment yaml spec

[kcpevey]

Feature description

Right now we only allow the GPU-specific environment variable. However, conda allows the setting of any env var inside an environment via the yaml spec - https://conda.io/projects/conda/en/latest/user-guide/tasks/manage-environments.html#setting-environment-variables

It would be nice to be able to set any environment variable.

Valid example:

channels:
  - conda-forge
dependencies:
  - pytorch
  - ipykernel
variables:
  CONDA_OVERRIDE_CUDA: "12.0"

Value and/or benefit

• Improve UX, reduce confusion when users see that the yaml spec has the option to add env vars
• Improve feature parity between conda-store and local conda usage (which is what users are comfortable with)

Anything else?

This was originally reported on nebari - nebari-dev/nebari#2445