Update `bin` path and other install fixes

This PR introduces multiple fixups for the broken installer.

Binaries path

The bin path for virtual env is now ../env/local/bin.

Here is one line in virtualenv tests that confirm this change: /tests/unit/discovery/py_info/test_py_info.py#L331

Robustify virtualenv installer

The recommended zipapp way to install the latest virtualenv is given here.

Fix __version__ injection

Using exec to import the version is anti-pattern, unpythonic, and fragile. What's more, it does not work, because the Python interpreter is looking in the CWD for the bashhub/version.py whereas it actually needs to look in the script location. The script location is found using the ugly method below:

import os
import sys

exec (open(os.path.join(os.path.dirname(sys.argv[0]),'./bashhub/version.py')).read())