Skip Xcode install on mac_mokey Android tests (flutter#163685)

These tests are failing on querying Xcode for simulator devices when
running Android tests. Ideally recipes wouldn't try to install Xcode
unless requested via `"sdk_version": "[xcode version number]"` property,
[but they do](flutter#163670). Since
the recipe[ mapping of macOS version to Xcode
version](https://flutter.googlesource.com/recipes/+/refs/heads/main/recipe_modules/osx_sdk/api.py#22)
is out of date, it's falling back to an old version of Xcode when none
are specified.

Stop installing Xcode on these builder configurations. If this works
I'll follow up and add it to `mac_pixel_7pro`, `mac_arm64_android`, etc.

First step of flutter#161655. If this
works `bringup` can be removed.
Fixes flutter#163309

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we expect every widget to implement].
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above.
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Author: jmagman

.ci.yaml

@@ -252,6 +252,10 @@ platform_properties:
       os: Mac-13|Mac-14
       cpu: x86
       device_type: "mokey"
+      $flutter/osx_sdk : >-
+        {
+          "skip_xcode_install": true
+        }
   mac_arm64_mokey:
     properties:
       dependencies: >-
@@ -262,6 +266,10 @@ platform_properties:
       os: Mac-13|Mac-14
       cpu: arm64
       device_type: "mokey"
+      $flutter/osx_sdk : >-
+        {
+          "skip_xcode_install": true
+        }
 
   mac_pixel_7pro:
     properties: