Rollup merge of #109667 - clubby789:fix-stage0-doc, r=ozkanonur

Always set `RUSTC_BOOTSTRAP` with `x doc`

Fixes #100060

Note that there is still a warning - the `unused_allocation` lint does not fire in stage 0, but that's just a matter of waiting for #104363 to land in beta

src/bootstrap/doc.rs

@@ -1027,10 +1027,11 @@ impl Step for RustcBook {
  if self.validate {
  cmd.arg("--validate");
  }
- if !builder.unstable_features() {
- // We need to validate nightly features, even on the stable channel.
- cmd.env("RUSTC_BOOTSTRAP", "1");
- }
+ // We need to validate nightly features, even on the stable channel.
+ // Set this unconditionally as the stage0 compiler may be being used to
+ // document.
+ cmd.env("RUSTC_BOOTSTRAP", "1");
+
  // If the lib directories are in an unusual location (changed in
  // config.toml), then this needs to explicitly update the dylib search
  // path.