* Instead of injecting our own dummy `build.rs` file, we will patch the
`Cargo.toml` files to specify a build script in the Nix store
* This allow cargo to notice the difference (i.e. changed build script
path) where it could not before (due to nix enforcing that all sources
always have the same timestamp)
* There are some edge cases where cargo will allow a lib.rs file to be
present but not a main.rs (e.g. a binary cannot be called "examples",
which will happen if the crate is called that and a main.rs file is
present)
