Consistently unset RUSTC_BOOTSTRAP when compiling bootstrap

Author: dtolnay

src/bootstrap/bootstrap.py

@@ -917,6 +917,8 @@ def build_bootstrap_cmd(self, env):
             if toml_val is not None:
                 env["{}_{}".format(var_name, host_triple_sanitized)] = toml_val
 
+        env.pop("RUSTC_BOOTSTRAP", None)
+
         # preserve existing RUSTFLAGS
         env.setdefault("RUSTFLAGS", "")