[ci] Temporarily disable ad_external_array on Metal (taichi-dev#7136)

This test is flaky. It does not fault out properly and CI detects a
success while a worker will be brought down by this crash, making CI
impossibly long.

Issue: #

### Brief Summary

tests/python/test_ad_external_array.py

@@ -5,7 +5,7 @@
 from tests import test_utils
 
 
-@test_utils.test()
+@test_utils.test(exclude=[ti.metal])
 def test_to_numpy():
     a = ti.field(dtype=float, shape=(), needs_grad=True)
     loss = ti.field(dtype=float, shape=(), needs_grad=True)
@@ -20,7 +20,7 @@ def func():
         0]
 
 
-@test_utils.test()
+@test_utils.test(exclude=[ti.metal])
 def test_from_numpy():
     a = ti.field(dtype=float, shape=(), needs_grad=True)
     loss = ti.field(dtype=float, shape=(), needs_grad=True)