[ENH] - Make the process of building GPU-enabled environments easier

[peytondmurray]

Feature description

Right now, to build GPU-enabled environments users must specify a specific environment variable as part of the environment.yaml that gets passed to conda-store; Currently this is the only environment variable that can be specified in this way; see #826 for a related issue about accepting any environment variable.

This makes for a pretty clunky user experience; this issue is to start the discussion around making this more pleasant for users.

Value and/or benefit

Making this feature less hidden will be a major improvement to users who depend on GPU environments.

Anything else?

No response