When setting up Rust toolchain manually, set default path as bazel-out/{toolchain_path}

[alexjpwalker]

When opening a project and loading a Rust file before running the appropriate Bazel build command - or when loading a Rust file at any time after having ever done the above - you'll see "No Rust toolchain configured".

I'm not sure how we could easily configure it to always try to auto-detect the Bazel-installed Rust toolchain in this case, but what we can definitely do, is modify the behaviour of the "Set up toolchain" option:

Currently it auto-detects the Rust toolchain in /Users/{user}/.cargo/bin, installed by rustup.

We should modify it to search the same paths that we've defined the Rust toolchain detection (on first load of a Rust file) to search; namely bazel-out for the Rust toolchain, and bazel-{projectName} for the stdlib sources.