Fix DeprecationWarning of conda.common.io.env_vars

In `conda_interface.py` we import `env_vars` from `conda.common.io`. This has been deprecated:
```
PendingDeprecationWarning: conda.common.io.env_vars is pending deprecation and will be removed in 27.3.
```
as of `conda 26.3.0` ([source](https://docs.conda.io/projects/conda/en/latest/release-notes.html#id29)).

This should be fixed before being removed.

Conda's own suggested replacement is `monkeypatch.setenv`/`delenv`, but that's for `pytest` and as far as I know it doesnt have the same callback parameter (to call `conda_replace_context_default` on enter/exit), and in our case `env_vars` is used in `fcp.py` in production and not in tests.

Start in conda_interface.py by tracing the deprecated env_vars import and then inspect its production use in fcp.py. Compare available environment-management replacements while preserving the callback that invokes conda_replace_context_default on entry and exit. Done means the deprecated import is removed without changing production behavior or producing the conda warning.

Written by the indexing model from the issue text.