Be explicit on pipx instructions

[dalonsoa]

To use a specific Python version with pipx in Linux and MacOS do: pipx install --python $(which python) muse_os.

[sgiarols]

It may be good to also add a note on how to install pipx. In fact, using brew as below

brew reinstall pipx
pipx ensurepath
pipx --python python3.9 MUSE-OS_

the installation does not proceed giving an error about the python version.

Alternatively, using pip to install pipx, is successful to install muse-os

[dalonsoa]

You need to give the full path to the python 3.9 executable, not just python3.9.