fix(tox): run each tox env on its own interpreter

[Moisan]

The tox commands call uv sync and uv run, which ignore the VIRTUAL_ENV that tox activates and use the project's .venv instead. Every tox env (py313, py314) therefore ran the unit tests on whichever interpreter built .venv (3.13 in CI), so 3.14 was never actually tested.

Point UV_PROJECT_ENVIRONMENT at the tox env dir so uv installs into and runs from the env tox created for that interpreter.

Evidence from CI

From the Test (py314) job on the latest main run (run 38946725826):

py314: commands[1]> uv run pytest --cov vunnel -v --durations=10 tests/unit/
warning: `VIRTUAL_ENV=.tox/py314` does not match the project environment path `.venv` and will be ignored
platform linux -- Python 3.13.16, pytest-9.1.1, pluggy-1.6.0 -- /home/runner/_work/vunnel/vunnel/.venv/bin/python
py314: OK

The job is labelled py314, but the tests run on 3.13.16 from .venv.

With this change, running tox -e py313 and tox -e py314 locally reports 3.13.16 from .tox/py313 and 3.14.8 from .tox/py314, and the unit tests pass in both.