Configures the spread setup to work with `snapcraft test`: - Comment out top-level `path`/`environment`/`include` so `snapcraft test` can manage them - Remove `type`/`allocate` stub from `craft` backend so `snapcraft test` can inject its own adhoc configuration - Switch snap install to use `$CRAFT_ARTIFACT` env var set by `snapcraft test` - Configure root SSH login in `cloud-config.yaml` for the `craft` backend (used by the LXD VM allocator) - Run `apt-get update` in `prepare` to ensure package lists are fresh Closes CRAFT-5063