Fix clippy lint

```
error: the borrowed expression implements the required traits
  --> libcnb-package/src/package.rs:67:24
   |
67 |         .manifest_path(&buildpack_directory.join("Cargo.toml"))
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: change this to: `buildpack_directory.join("Cargo.toml")`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrows_for_generic_args
   = note: `-D clippy::needless-borrows-for-generic-args` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::needless_borrows_for_generic_args)]`

error: could not compile `libcnb-package` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
Error: Process completed with exit code 101.
```

libcnb-package/src/package.rs

@@ -64,7 +64,7 @@ fn package_libcnb_buildpack(
     destination: &Path,
 ) -> Result<(), PackageLibcnbBuildpackError> {
     let cargo_metadata = MetadataCommand::new()
-        .manifest_path(&buildpack_directory.join("Cargo.toml"))
+        .manifest_path(buildpack_directory.join("Cargo.toml"))
         .exec()
         .map_err(PackageLibcnbBuildpackError::CargoMetadataError)?;