Intermediate build not generating targets

[deifactor]

Using the latest version, running nix build . appears to generate a deps output with an empty target dir; /nix/store/3q39kpl5441cdi8w6ldna9f8dlmxq9wh-blah-deps-0.1.0 has a zstd file that's only 93 bytes or so.

Using df71f5e produces an actual deps output. So I think 08afb3d broke it, but I don't know enough about nix or cargo build internals to figure out why CARGO_TARGET_DIR isn't working.

[deifactor]

Aha, I figured it out. It turns out that --out-path and CARGO_TARGET_DIR have different behavior.

--out-path puts the binary in the specified directory but leaves the intermediates in target. CARGO_TARGET_DIR moves everything. So I think changing the copyTarget code to look in out instead of target will fix it.