chore: use uv-version in uv-virtualenv (#3221)
## Summary
This is mainly a cleanup PR to leverage uv-version in uv-virtualenv
instead of passing it via `uv`.
In #1852 I introduced the ability to pass extra cfg to `gourgeist` for
the primary purpose of passing the uv version, but since the dawn of the
uv-version crate dynamically passing more values to pyvenv.cfg is no
longer needed.
## Test Plan
Existing `uv` tests should still verify `uv = <version>` exists in the
venv and make sure no regressions were introduced.
