Add explicit cipd credentials to mac_toolchain for official buildbot.

Buildbot is being turned down but is not fully dead yet.

Bug: 986488
Change-Id: Ibc44a0ebfe82d5f2949ddf7f62de4eefe778617c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713869
Commit-Queue: Erik Chen <erikchen@chromium.org>
Commit-Queue: Michael Moss <mmoss@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679878}

build/mac_toolchain.py

@@ -152,6 +152,14 @@ def InstallXcodeBinaries():
   args = [
       'cipd', 'ensure', '-root', binaries_root, '-ensure-file', '-'
   ]
+
+  # Buildbot slaves need to use explicit credentials. LUCI bots should NOT set
+  # this variable. This is temporary code used to make official Xcode bots
+  # happy. https://crbug.com/986488
+  creds = os.environ.get('MAC_TOOLCHAIN_CREDS')
+  if creds:
+    args.extend(['--service-account-json', creds])
+
   p = subprocess.Popen(
       args, stdin=subprocess.PIPE, stdout=subprocess.PIPE,
       stderr=subprocess.PIPE)