fix(ci): formal-verification creusot-verify cannot apt-install alt-ergo #250

Split out from the 2026-06-13 checkpoint (#238); surfaced by #247's broad CI run.

Symptom

.github/workflows/formal-verification.yml job creusot-verify fails at its install step:

sudo apt-get install -y --no-install-recommends why3 z3 alt-ergo
E: Package 'alt-ergo' has no installation candidate
##[error]Process completed with exit code 100.

why3 and z3 install fine; alt-ergo is not in the runner's apt. Pre-existing (fails on main, independent of any PR diff).

Fix

Creusot needs Alt-Ergo as a solver backend, so install it via opam (opam install alt-ergo) rather than apt, or pin a source/binary. Then drop alt-ergo from the apt line.

Tracker: #238