[infra] Run pkg/(front_end|kernel) tests only on front-end builders

They are rather slow, and there is no point in running them twice.

Change-Id: Ibdf8de1d7420e89b8287652b958a4f7e67b1de86
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/97933
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>

tools/bots/test_matrix.json

@@ -1642,7 +1642,7 @@
           "name": "package unit tests",
           "arguments": [
             "-nunittest-asserts-${mode}-${system}",
-            "pkg"
+            "pkg/pkg/(?!front_end/|kernel/)"
           ]
         },
         {
@@ -1676,7 +1676,7 @@
           "name": "package unit tests",
           "arguments": [
             "-nunittest-asserts-${mode}-${system}",
-            "pkg"
+            "pkg/pkg/(?!front_end/|kernel/)"
           ]
         },
         {