Replace pip with uv pip

[jtraglia]

I've been wanting to switch to uv pip for a while now. It's a lot faster. This will pip install uv (in the venv) once when creating the virtual environment. It also rebuilds eth2spec now everytime we build the pyspec, because it's so fast.

Some screenshots with numbers:

Before:

From clean build:

From existing build:

After:

From clean build:

From existing build: