Copy /target directory; avoid rebuilding from scratch in each scratch dir

[sourcefrog]

Today, cargo-mutants does a from-scratch build in each scratch directory. In fact, it doesn't even copy /target because I found that even when it was copied, we still built from scratch, so the copy is just a waste of time.

I'm not sure what is causing cargo to rebuild.

I already tried setting file mtimes and that does not seem to be enough.

@jeremyBanks suggested using --remap-path-prefix like in https://github.com/denoland/wasmbuild/pull/136/files and that sounds very promising but I do not yet have it working.

https://github.com/rust-lang/cargo/issues/2904 has some information on how to get trace output from cargo explaining why it rebuilds things.