Run Mac x64 build tests in postsubmit only (#142334)

It shouldn't be very common for x64 example projects to not build if they built on arm64.  We have other tool tests that would probably catch those issues in presubmit.

Instead run them on postsubmit only to free up limited x64 staging resources.

Follow up from #141206

.ci.yaml

@@ -3193,6 +3193,7 @@ targets:
 
   - name: Mac_x64 build_tests_1_4
     recipe: flutter/flutter_drone
+    presubmit: false # Rely on Mac_arm build_tests in presubmit.
     timeout: 60
     properties:
       add_recipes_cq: "true"
@@ -3211,6 +3212,7 @@ targets:
 
   - name: Mac_x64 build_tests_2_4
     recipe: flutter/flutter_drone
+    presubmit: false # Rely on Mac_arm build_tests in presubmit.
     timeout: 60
     properties:
       add_recipes_cq: "true"
@@ -3229,6 +3231,7 @@ targets:
 
   - name: Mac_x64 build_tests_3_4
     recipe: flutter/flutter_drone
+    presubmit: false # Rely on Mac_arm build_tests in presubmit.
     timeout: 60
     properties:
       add_recipes_cq: "true"
@@ -3247,6 +3250,7 @@ targets:
 
   - name: Mac_x64 build_tests_4_4
     recipe: flutter/flutter_drone
+    presubmit: false # Rely on Mac_arm build_tests in presubmit.
     timeout: 60
     properties:
       add_recipes_cq: "true"