`VIRTUAL_ENV` takes precedence over `CONDA_PREFIX` (#2574)
It is a common pattern to have an active conda base env (that sets
`CONDA_PREFIX`) and then create a venv on top of that (setting
`VIRTUAL_ENV`).
Previously, we would error when both `VIRTUAL_ENV` and `CONDA_PREFIX`
were set, now `VIRTUAL_ENV` takes precedence over `CONDA_PREFIX`.
Fixes #2028
