message: Remove unhelpful link to Cargo book
  It didn't have any information about RUSTC_BOOTSTRAP itself, only the
  general `rustc-env` feature.
diff: @@ -589,8 +589,7 @@ impl BuildOutput {
  // behavior, so still only give a warning.
  if nightly_features_allowed {
  warnings.push(format!("Cannot set `RUSTC_BOOTSTRAP={}` from {}.\n\
  - note: Crates cannot set `RUSTC_BOOTSTRAP` themselves, as doing so would subvert the stability guarantees of Rust for your project.\n\
  - help: See https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-env for details.",
  + note: Crates cannot set `RUSTC_BOOTSTRAP` themselves, as doing so would subvert the stability guarantees of Rust for your project.",
  val, whence
  ));
  } else {
  @@ -598,8 +597,7 @@ impl BuildOutput {
  // Abort with an error.
  anyhow::bail!("Cannot set `RUSTC_BOOTSTRAP={}` from {}.\n\
  note: Crates cannot set `RUSTC_BOOTSTRAP` themselves, as doing so would subvert the stability guarantees of Rust for your project.\n\
  - help: If you're sure you want to do this in your project, set the environment variable `RUSTC_BOOTSTRAP={}` before running cargo instead.\n\
  - help: See https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-env for details.",
  + help: If you're sure you want to do this in your project, set the environment variable `RUSTC_BOOTSTRAP={}` before running cargo instead.",
  val,
  whence,
  pkg_name,
  