Always set RUSTC_BOOTSTRAP with x doc

Author: clubby789

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.