micromamba paths

[Hind-M]

From prefix discord:

• Micromamba 2 shell activation puts non-existent dirs on Linux $PATH. It should check that dirs exist before adding them to $PATH.
  One line at fault is export PATH="${MAMBA_ROOT_PREFIX}/condabin:${PATH} where condabin does not exist here.
  I have $MAMBA_ROOT_PREFIX/bin holding micromamba itself, and my own login profile scripts already place it on $PATH and define $MAMBA_EXE. My base environment is otherwise empty.

• I also had a bothersome libmamba error message and made an empty $MAMBA_ROOT_PREFIX/conda-meta dir just to stop it.

• Lastly, I named $MAMBARC as 'condarc' because micromamba seems to prefer conda names (I'm not sure).