fix crates.io install #140
Description
the crates.io installation requires libdrift is installed locally (CARGO_DRIFT_FFI_PATH=...).
it should work out of the box but the crates.io install doesn't include the drift-ffi-sys source code (since it includes a Cargo.toml crates.io won't publish it)
--- stderr
Warning: can't set `imports_granularity = Crate`, unstable features are only available in nightly channel.
Error: Os { code: 2, kind: NotFound, message: "No such file or directory" }