rustpkg: Install to RUST_PATH

Install to the first directory in the RUST_PATH if the user set a
RUST_PATH. In the case where RUST_PATH isn't set, the behavior
remains unchanged.

Closes #7402
auto merge of #9147 : catamorphism/rust/rustpkg-install-to-rust-path, r=catamorphism,metajack

r? @metajack Install to the first directory in the RUST_PATH if the user set a
RUST_PATH. In the case where RUST_PATH isn't set, the behavior
remains unchanged.

Closes #7402