Avoid spawning conda to get the value for the env variable `CONDA_SHLVL`

[DonJayamanne]

The value for this is defined in etc/profile.d/conda.sh (csh) file of the conda installtion folder.

This env variable is used in the env variables for terminals.

[eleanorjboyd]

@DonJayamanne is this still valid with the envs ext and PET work?

[DonJayamanne]

PET doesn't return CONDA_SHLVL, perhaps something that @anthonykim1 @karthiknadig might be able to answer.
I.e. whether we're still spawning conda to get this info