Globally set RUSTC_BOOTSTRAP

This is a beta-only commit to avoid problems introduced by #97277; we will want
a more targeted patch that specifically re-enables this for a subset of the
books (nomicon, unstable book, ...?) and testing via a stable-branch builder for
tools, but that isn't worth backporting.

src/bootstrap/bootstrap.py

@@ -1067,6 +1067,7 @@ def bootstrap(help_triggered):
     env = os.environ.copy()
     env["BOOTSTRAP_PARENT_ID"] = str(os.getpid())
     env["BOOTSTRAP_PYTHON"] = sys.executable
+    env["RUSTC_BOOTSTRAP"] = '1'
     run(args, env=env, verbose=build.verbose, is_bootstrap=True)