mamba: `micromamba shell` broken since 1.4.5

I think this change https://github.com/mamba-org/mamba/pull/2527 broke micromamba shell

With 1.4.4, if I do

micromamba shell -n ...
which python

I get the environment’s Python.

With 1.4.5, I get the system wide Python. Probably something off with PATH.

cc @AntoinePrv

fyi @0xbe7a