Enforce cpu explicitly for Mac devicelab test beds (#101871) (#102685)

Co-authored-by: keyonghan <54558023+keyonghan@users.noreply.github.com>
diff --git a/.ci.yaml b/.ci.yaml
index 1bf7e08..240be0f 100755
--- a/.ci.yaml
+++ b/.ci.yaml
@@ -85,7 +85,7 @@
         []
       os: Mac-12
       device_type: none
-      mac_model: Macmini8,1
+      cpu: x86
       xcode: 13a233
   mac_android:
     properties:
@@ -106,6 +106,7 @@
           {"dependency": "open_jdk", "version": "11"}
         ]
       os: Mac-12
+      cpu: x86
       device_os: N
   mac_ios:
     properties:
@@ -127,6 +128,7 @@
           {"dependency": "ios_signing"}
         ]
       os: Mac-12
+      cpu: x86
       device_os: iOS-15.1
       xcode: 13a233
   windows: