Use `uv pip` instead of `pip` (or optionally support it)

`pip install` is used here: https://github.com/heroku/buildpacks-python/blob/0bcbf989c8f473e70345829003f4447c0eabf4a3/src/layers/pip_dependencies.rs#L81-L89.

It would be great to also / alternatively use `uv pip install`. This is intended to be a drop-in replacement for `pip`, but with much faster dependency resolution and other optimizations. Ref: https://docs.astral.sh/uv/pip/

In practice, I notice a decent speed up when using `uv pip install` over `pip install`, and it would be a great improvement to have this speedup in Heroku's Python buildpack.

I think it probably makes sense as an opt-in feature with an environment variable or similar, since there's no way to distinguish when to use uv pip or pip based on the file structure alone.