build: Move build/toolchain/apple/linker_driver.py to Python3.

The script seems to be compatible with Python3 already, so this CL
updates its hashbang line.

Bug: 942720
Change-Id: I630dc83b756ad7559f1def1bfb127d14c1028b0f
Tested: Local build on macOS 12.3, which lacks /usr/bin/python.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3425461
Auto-Submit: Victor Costan <pwnall@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/main@{#964888}

build/toolchain/apple/linker_driver.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be