use conda --freeze-installed in Deps.add()

According to the conda docs, we should be able to use this option to prevent already-installed packages from changing. We could use this in Deps.add() to guarantee consistent state.

However, the option doesn't seem to work! In a fresh environment, doing `conda install python` then `conda install --freeze-installed numpy` installs Python 3.10 then downgrades it to Python 3.9!