contracts-fixtures: Do not assume that rustup is installed (#2586)

The build script was assuming that `rustup` is installed, which breaks
the build on systems that do not use `rustup`. This pull request just
fixes it by not panicking on the call to `rustup`.

substrate/frame/contracts/fixtures/build.rs

@@ -147,9 +147,7 @@ fn invoke_cargo_fmt<'a>(
  if !Command::new("rustup")
  .args(&["run", "nightly", "rustfmt", "--version"])
  .output()
- .expect("failed to execute process")
- .status
- .success()
+ .map_or(false, |o| o.status.success())
  {
  return Ok(())
  }
@@ -169,10 +167,12 @@ fn invoke_cargo_fmt<'a>(
  let stderr = String::from_utf8_lossy(&fmt_res.stderr);
  eprintln!("{}\n{}", stdout, stderr);
  eprintln!(
- "Fixtures files are not formatted.\nPlease run `rustup run nightly rustfmt --config-path {} {}/*.rs`",
+ "Fixtures files are not formatted.\n
+ Please run `rustup run nightly rustfmt --config-path {} {}/*.rs`",
  config_path.display(),
  contract_dir.display()
  );
+
  anyhow::bail!("Fixtures files are not formatted")
 }