message: support relative XARGO_RUST_SRC
diff: @@ -257,7 +257,10 @@ fn setup(subcommand: MiriCommand) {
  // Determine where the rust sources are located. `XARGO_RUST_SRC` env var trumps everything.
  let rust_src = match std::env::var_os("XARGO_RUST_SRC") {
  - Some(val) => PathBuf::from(val),
  + Some(val) => {
  + let val = PathBuf::from(val);
  + val.canonicalize().unwrap_or(val)
  + }
  None => {
  // Check for `rust-src` rustup component.
  let sysroot = miri()
  