Support environment.yml variables

[jacobwilliams]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

Conda environment.yml files can support environment variables like so:

variables:
  HELLO: 1

that get added to the environment when it is activated. It seems that these are not preserved when using conda-lock to generate the lock file? It would be nice to include them.

Why is this needed?

Would make conda-lock more useful for this important use case.

What should happen?

It should 'just work' (variables are carried over to the lock files and used when the env is created). Also they should be inserted into the yml files generated by the --kind env option.

Additional Context

No response