racer expects SRC_RUST_PATH environment variable, which varies depending on override

`racer` wants a folder with the `rust` source in it to provide "go to definition" and autocompletion on built-ins. Unfortunately, those vary based on the Rust version.

Some possible work-arounds:
- Maintain an `src` folder based on the current default override.
- Provide `multirust run (stable|beta|nightly) racer` with the path to the right `src` folder passed in as an environment variable.

Start by reading the multirust command path involved in `multirust run` and how it sets environment variables for the selected Rust version. Check how `racer` consumes `SRC_RUST_PATH`; done means selecting the matching Rust source directory for stable, beta, and nightly without requiring a manually maintained override.

Written by the indexing model from the issue text.