Fix INSTALL instructions for newer versions of pip

[jrom99]

Split from #400

I updated the INSTALL file to reflect changes to pip install . since pip23 and pip24, where all arguments must be in a key=value form. I took the liberty to change how some optional dependencies can be set/unset to be more explicit (--no-libxml is now --libxml=false), and as far as I've checked, they are not used in the current github workflow.

I couldn't find how to set PYMOL_PATH in a way that pip install . is able to listen:

• Raises unknown argument error:
  pip install . --config-settings pymol-path=/some/path
pip install . --config-settings pymol_path=/some/path
• Ignored (pymol is placed in venvprefix/bin/pymol or pythonuserbase/bin/pymol):
  PYMOL_PATH=/some/path/ pip install .
env PYMOL_PATH=/some/path/ pip install .