bootstrap: use RUSTC_BOOTSTRAP in distcheck

Author: notriddle

src/bootstrap/test.rs

@@ -2514,6 +2514,9 @@ impl Step for Distcheck {
         let toml = dir.join("rust-src/lib/rustlib/src/rust/library/std/Cargo.toml");
         builder.run(
             Command::new(&builder.initial_cargo)
+                // Will read the libstd Cargo.toml
+                // which uses the unstable `public-dependency` feature.
+                .env("RUSTC_BOOTSTRAP", "1")
                 .arg("generate-lockfile")
                 .arg("--manifest-path")
                 .arg(&toml)