environment type "system" ignores "python" setting

[sumpfralle]

1. minimal pyproject.toml file:

   [tool.hatch.envs.default]
   python = "/usr/bin/python3"
   type = "system"

2. create environment: hatch --verbose env create default:

   Finished creating environment: default
   /usr/bin/python: No module named pip
   Finished installing project in development mode
   

I think, there are three issues here:

• A) I guess, hatch should break with an error, if pip is unavailable.
• B) hatch.env.system does not use the python setting of the environment.
  • Maybe this is intended? Only the virtual environment is documented at the moment. Thus, it is unclear, whether the python setting should be supported by the system environment, too. I just expected it here.
• C) hatch prefers /usr/bin/python over /usr/bin/python3
  • Here on my Debian system /bin/bin/python is still pointing to Python2. This is a valid setup according to PEP-394.

This issue is related to #583. Maybe not all aspects of that issue were fixed by 90a77ef.

Regarding (C): I would suggest to reverse the order of search paths in env.plugin.interface (system_python).