Make self-update an opt-in Cargo feature (#2606)
## Summary
Ensures that (e.g.) installs from conda-forge, Homebrew, and other
distributions don't expose `uv self update` at all.
We'll still show `uv self update` for `pip install uv`, but it will fail
with a good error. Removing the `uv self update` from `pip`-installed
`uv` is more complicated, since we'd need to build separately for the
installer vs. for PyPI.
Closes #2588.
