I've been looking at UV, and wondered if we could get a speed up with virtualenv/venv by checking for pip in build's environment. If pip is present and of reasonable version, we could skip installing it into the isolated environments and use --python to run the outer pip command against the inner environment. That saves at least 200ms per environment, and maybe a lot more if pip has to be downloaded. If pip is not present, then we do what we do now.

Thoughts?