This is needed particularly when a new interpreter is used,
via -p/--python. Re-execing the same `virtualenv.py` will generally lead
to its path being added to the start of sys.path (as usual). And usually
its path will be the site-packages of the previous interpreter. This
will lead to issues if older backported packages are present in the
old environment (which will then get preference being imported).
Should fix #779, #774, #763