maturinBuildHook: use dist dir relative to cargoRoot

With `cargoRoot` set to a subdirectory of the source, where the
Cargo.{lock,toml} are found, the final mv would previously fail, since
the build results appear relative to cargoRoot, not to the original
build directory.

pkgs/build-support/rust/hooks/maturin-build-hook.sh

···
 
     # Move the wheel to dist/ so that regular Python tooling can find it.
     mkdir -p dist
-    mv target/wheels/*.whl dist/
+    mv ${cargoRoot:+$cargoRoot/}target/wheels/*.whl dist/
 
     # These are python build hooks and may depend on ./dist
     runHook postBuild