Fix issue #13 regarding duplicate toolchain artifact URLs.

Some distinct toolchain artifacts may share the same download URL (e.g., .../rust-docs-nightly-x86_64-unknown-linux-gnu.tar.xz is shared across several other processor variants). Before version 0.3.0, this was handled naturally by the sequential nature of the download operation, but the new asynchronous support from 0.3.0 failed to account for the possible duplication.