conda fix

[maxim-uvarov]

seems like that somehow #a61256d broke conda, which I use:

   ╭─[/Users/user/apps-files/github/nu_scripts/modules/virtual_environments/conda.nu:85:1]
 85 │
 86 │     $env.PROMPT_COMMAND = if $env.CONDA_OLD_PROMPT_COMMAND == $nothing {
    ·                              ──────────────┬──────────────
    ·                                            ╰── expected operator

Seems like this PR fix it.

[Hofer-Julian]

Thanks!