Use xcode 12a8189n instead of 11e146.

After https://chromium-review.googlesource.com/c/chromium/src/+/2391798,
some WebRTC chromium.webrtc.fyi bots have started to fail because of
an #import error on ScreenTime.h (see [1]).

This CL updates these bots to use the same xcode build used by Chromium.

[1] - https://ci.chromium.org/p/chromium/builders/webrtc.fyi/WebRTC%20Chromium%20FYI%20ios-device/7252

Bug: 1123704
Change-Id: I5cc8d0af6b41ef81725d96dbce0f1854f15eed6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391155Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804704}

ios/build/bots/chromium.webrtc.fyi/WebRTC Chromium FYI ios-device.json

@@ -3,7 +3,7 @@
     "Builder for 64-bit devices.",
     "Build is performed with gn+ninja."
   ],
-  "xcode build version": "11e146",
+  "xcode build version": "12a8189n",
   "gn_args": [
     "goma_dir=\"$(goma_dir)\"",
     "ios_enable_code_signing=false",

ios/build/bots/chromium.webrtc.fyi/WebRTC Chromium FYI ios-simulator.json

@@ -2,7 +2,7 @@
   "comments": [
     "Runs tests on @3x, @2x, 64-bit phone, tablet, iOS 12."
   ],
-  "xcode build version": "11e146",
+  "xcode build version": "12a8189n",
   "gn_args": [
     "goma_dir=\"$(goma_dir)\"",
     "ios_enable_code_signing=false",