Fixes #5535 venv powershell infinite prompt

[mjvankampen]

Changelog: Bugfix: Make prompt names unique when using multiple virtualenv scripts in Powershell.
Docs: Omit

Fixes #5535

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[memsharded]

This is looking good so far.

One thing I would try to implement (not for this PR, just for us, later), is when this becomes a customizable generator, to be able to give it a custom name, not just the "virtualenv", "virtualrunenv" name.

[jgsogo]

LGTM!