#2161 setup: an interrupted engine update is tried again instead of skipped for good (#2159)

Description

Fixes #2159.

`get_prebuilt` moves `BUILD.json` to `BUILD.json.prev` before downloading, and `update_installed_engine` only puts it back if the process survives to its `new is None` branch. After a crash, reboot or closed window mid-update, every later update returned early on the missing `BUILD.json`, so the install stayed on its old engine without saying so.

Now, when `BUILD.json` is missing but `BUILD.json.prev` and the exe are there, the stamp is restored (with a warning) and the update runs again.

- New test `test_an_interrupted_update_is_tried_again` (fails before the change, passes after).
- `tools/` suite: 3 failures in `test_setup_engine_hash.WhatARefusalDoesToTheCaller`, which fail the same way on `main` without this change.