[test] Bump shards on slow builder

TBR=mslekova@chromium.org

No-Try: true
Change-Id: Ib39993d6c82d499d8dc0f278cff4ec7dbfb345d0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2036075Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66098}

infra/testing/builders.pyl

@@ -1417,7 +1417,7 @@
       {'name': 'mozilla'},
       {'name': 'test262', 'variant': 'default'},
       {'name': 'v8testing', 'shards': 3},
-      {'name': 'v8testing', 'variant': 'extra'},
+      {'name': 'v8testing', 'variant': 'extra', 'shards': 2},
       {'name': 'v8testing', 'variant': 'trusted'},
     ],
   },