bootstrap.py: build bootstrap binary with --features=tracing if BOOTSTRAP_TRACING env var is set

Author: jieyouxu

src/bootstrap/bootstrap.py

@@ -1129,6 +1129,10 @@ def build_bootstrap_cmd(self, env):
             "-Zroot-dir=" + self.rust_root,
         ]
         args.extend("--verbose" for _ in range(self.verbose))
+
+        if "BOOTSTRAP_TRACING" in env:
+            args.append("--features=tracing")
+
         if self.use_locked_deps:
             args.append("--locked")
         if self.use_vendored_sources: