inherit the main crate's meta when building with tests

[jrobsonchase]

When runTests = true, the mainProgram of the crate being built/tested gets lost, preventing you from running the crate with nix run:

$ nix run .#scratch
error: unable to execute '/nix/store/707g4alfpbpxxpdr5zaxay4j0mvdsfxr-rust_scratch-0.1.0-linked/bin/rust_scratch': No such file or directory

Bringing in the original meta in the -linked package resolves the problem:

$ nix run .#scratch
Hello, world!