[CFE] Fix weekly but needing dart2js_platform_unsound.dill

The weekly bot currently fails because the weak suite no longer have
dart2js_platform_unsound.dill available. This CL fixes that.

Change-Id: I2a8e71b6b8ce1ec9a9aa3d9456245c217f8ac26d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/404102
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

tools/bots/test_matrix.json

@@ -3476,7 +3476,8 @@
             "--mode=release",
             "--arch=x64",
             "create_sdk",
-            "ddc_stable_unsound_sdk"
+            "ddc_stable_unsound_sdk",
+            "dart2js_platform_unsound.dill"
           ]
         },
         {