Install pip to fix test vector generators

[jtraglia]

Since upgrading to python v3.12, pip is not longer installed by default. We must install it ourselves.

See this error in the test vector generation action:

https://github.com/ethereum/consensus-specs/actions/runs/10895315028/job/30233349401

The error messages are long, but this is the important part:

venv/bin/python3: No module named pip

We cannot add pip to the test generator requirements, as it's needed before then. So we must install pip in another step. An easy way to do this is to install the "preinstallation requirements" which includes pip.

[jtraglia]

This isn't enough to fix things. Closing. Will make a new PR.