Avoid accidentally enabling unstable features in compilers (take 2)

This allows rustbuild to control whether crates can use nightly features or not.
It also prevents rustbuild from using nightly features itself.

src/bootstrap/bootstrap.py

@@ -1171,7 +1171,6 @@ def bootstrap(help_triggered):
     env = os.environ.copy()
     env["BOOTSTRAP_PARENT_ID"] = str(os.getpid())
     env["BOOTSTRAP_PYTHON"] = sys.executable
-    env["RUSTC_BOOTSTRAP"] = '1'
     if build.rustc_commit is not None:
         env["BOOTSTRAP_DOWNLOAD_RUSTC"] = '1'
     run(args, env=env, verbose=build.verbose, is_bootstrap=True)