Dramatically simplified build setup based on new stagex release.

Reproduces with:
sha256:85569c261e7e97a0a3fa71dd95f220a60fd310fdee0ddf321801430db0b36c8e

Tested reproduction across one intel xeon machine and one threadripper and mystenlabs su-operations docker build: https://github.com/MystenLabs/sui-operations/actions/runs/13466799440/job/37634191938