message: clarify error message when sysroot was not found
diff: @@ -200,7 +200,8 @@ fn find_sysroot() -> String {
  _ => {
  option_env!("RUST_SYSROOT")
  .expect(
  - "need to specify RUST_SYSROOT env var or use rustup or multirust",
  + "Could not find sysroot. Either set MIRI_SYSROOT at run-time, or at \
  + build-time specify RUST_SYSROOT env var or use rustup or multirust",
  )
  .to_owned()
  }
  