Go through rustup proxy if possible

build.py

@@ -239,8 +239,12 @@ def build_cpp_src(targets: set[str]):
 def run_cargo(cmds: list[str]):
     ensure_paths()
     env = os.environ.copy()
-    env["PATH"] = f"{rust_sysroot / "bin"}{os.pathsep}{env["PATH"]}"
     env["CARGO_BUILD_RUSTFLAGS"] = f"-Z threads={min(8, cpu_count)}"
+    if shutil.which("rustup"):
+        # Go through rustup proxies by default if available
+        env["RUSTUP_TOOLCHAIN"] = str(rust_sysroot)
+    else:
+        env["PATH"] = f"{rust_sysroot / "bin"}{os.pathsep}{env["PATH"]}"
         # Cargo calls executables in $RUSTROOT/lib/rustlib/$TRIPLE/bin, we need
         # to make sure the runtime linker also search $RUSTROOT/lib for libraries.
         # This is only required on Unix, as Windows search dlls from PATH.