BUG,UBY: venv: deactivate resets WORKON_HOME to the default

[Wes Turner]

venv is setting WORKON_HOME to the default set in venv.py.

Instead:

• venv could set WORKON_HOME to os.environ.get('WORKON_HOME', opts.WORKON_HOME if opts.WORKON_HOME else default_workon_home) when: WORKON)

cases:

• using venv.py for a prefix template
• using venv.py for a virtualenv
• using venv.py for a conda env
• using venv.py with -E/--from-environ