Make interactions with `conda` consistent

[soapy1]

Some conda self commands use the conda executable to manage packages in the base environemnt and some use the conda api.

For example, the conda self update command shells out to the conda executable: https://github.com/jaimergp/conda-self/blob/main/conda_self/install.py.

Whereas all the other conda self commands use the conda api. For example conda self install https://github.com/jaimergp/conda-self/blob/main/conda_self/cli/main_install.py.

It would be good to be consistent in how conda self interacts with conda.