Fix relocatable nushell activation script (#17036)
<!--
Thank you for contributing to uv! To help us out with reviewing, please
consider the following:
- Does this pull request include a summary of the change? (See below.)
- Does this pull request include a descriptive title?
- Does this pull request include references to any relevant issues?
-->
## Summary
<!-- What's the purpose of the change? What does it do, and why? -->
Nushell activation now computes the venv root dynamically via path self
for relocatable venvs, while non-relocatable venvs still embed a quoted
absolute path
Still keep `activate.csh` (maybe delete is also an option)
close https://github.com/astral-sh/uv/issues/16973
<!-- How was it tested? -->
