Limit pointer compression builder to only run the default shipping config tests

Bug: v8:11454
Change-Id: I44ed59d5ca7d998ff1affd8acb30862b532a8b3b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2759519
Auto-Submit: Almothana Athamneh <almuthanna@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73411}

infra/testing/builders.pyl

@@ -577,7 +577,7 @@
       'os': 'Ubuntu-16.04',
     },
     'tests': [
-      {'name': 'v8testing', 'shards': 12},
+      {'name': 'v8testing', 'variant': 'default', 'shards': 4},
     ],
   },
   ##############################################################################