Conda.bin_dir gives wrong directory.

`Conda.bin_dir(:uniref)` gives `"/Users/jakobnissen/miniconda3/envs/uniref/envs/uniref/bin"` - note the `envs/uniref` is duplicated.

This probably comes from the fact that ROOTENV is `"/Users/jakobnissen/miniconda3/envs/uniref"`, and `bin_dir` is defined by joining `prefix` with `bin`, and `prefix` is defined as joining ROOTENV with `envs/uniref`.

And I think *that* ultimately comes from the README.md saying that you can use an existing environment by doing:
`ENV["CONDA_JL_HOME"] = "/Users/jakobnissen/miniconda3/envs/my_env"`, when in fact you should be omitting the `envs/my_env` at the end.

Review README.md and the definitions of bin_dir and prefix, starting with how ROOTENV and CONDA_JL_HOME are combined for an existing environment. Confirm the expected environment path and verify that Conda.bin_dir(:uniref) no longer duplicates envs/uniref.

Written by the indexing model from the issue text.