Add `CACHEDIR.TAG` to uv-created virtualenvs (#1653)
## Summary
Just as we mark virtualenvs as `gitignore`d by default, we should also
mark them as `CACHEDIR.TAG`, to ensure that they aren't included in
backups, etc.
Closes https://github.com/astral-sh/uv/issues/1648.
## Test Plan
Ran `cargo run venv` and:
```
❯ ls .venv
CACHEDIR.TAG bin lib pyvenv.cfg
```
