The seatbelt leg of the sandbox-e2e matrix also ran `pnpm run test`, and the
complete unit suite takes 16 to well over 20 minutes on macos-latest, so the
shared 20-minute job budget cancelled the leg on most master pushes — before
the Seatbelt confinement e2e ever ran. Move the parity run into a `unit-darwin`
job with a 45-minute budget; the four e2e legs keep 20 minutes.