[red-knot] Change venv discovery (#17099)
## Summary
Rewrites the virtual env discovery to:
* Only use of `System` APIs, this ensures that the discovery will also
work when using a memory file system (testing or WASM)
* Don't traverse ancestor directories. We're not convinced that this is
necessary. Let's wait until someone shows us a use case where it is
needed
* Start from the project root and not from the current working
directory. This ensures that Red Knot picks up the right venv even when
using `knot --project ../other-dir`
## Test Plan
Existing tests, @ntBre tested that the `file_watching` tests no longer
pick up his virtual env in a parent directory
