The Worker-started source build used the VM scripts, provision script and manifest pins embedded in
the Worker that started it, so a golden built from a newer commit could miss that commit's new VM
scripts (seen live: snap_HTsU from 19f900e has #122's orcad but not orca-desktop-proxy or
orca-desktop-page.js). source-build.sh now sources image/commit-provision-env.mjs from the fetched
checkout (no dependencies; mirrors provisionEnv, enforced by a test), so the image ships exactly that
commit. Tests also require every image/vm file to be a declared VM script and installed by provision.
