Consider adding 'target-dir' option about cargo-deb

[kornelski]

This is the same as setting CARGO_TARGET_DIR env var. cargo deb should work with the env var.

However, I don't think this has any special relationship with cargo clean. If you call cargo clean without setting the same target dir (whether via env var or the arg), you're just making it irrelevant and act on a directory that probably doesn't exist anyway.

OTOH if you set different target dirs for different builds, then there's no reuse. If you set the same target dir for different targets, I'm not sure if there's any reuse happening either. It can't reuse any target-specific code of course. Maybe some host build dependencies that don't use build.rs could be reused? (the build script can inspect the target and alter crate's behavior, so such crates aren't reusable across targets). But given that libc has a build.rs, there's very little that can be reused.