prepare-ex command has extreme latency due to null registry updates

[brson]

Each call to cargo generate-lockfile does a registry update, so this step takes a great amount of time. Cargo itself does not expose a way to avoid this. It is possible to do with the cargo API, but I'd prefer to just use cargo from the command line if possible.

cc rust-lang/cargo#3479

[Eh2406]

I think; The line is https://github.com/rust-lang-nursery/crater/blob/28c2e3c74520b9beeab5af781cc5f6cc75815437/src/ex.rs#L460
and cargo ... -Z offline or airplane or something is on its way. So this is almost doable.