[infra] Rebalance Linux ARM64 JIT versus AOT shards.

Change-Id: I4bba3ec82baade734b7c24d7234ffa5b2747796d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/506503
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

tools/bots/test_matrix.json

@@ -1588,7 +1588,7 @@
             "vm/dart/tsan"
           ],
           "fileset": "vm",
-          "shards": 3
+          "shards": 4
         }
       ]
     },
@@ -1781,7 +1781,7 @@
             "pkg/pkg/vm_service/"
           ],
           "fileset": "vm",
-          "shards": 6
+          "shards": 4
         }
       ]
     },
@@ -1811,7 +1811,7 @@
             "pkg/pkg/vm_service/"
           ],
           "fileset": "vm",
-          "shards": 6
+          "shards": 8
         }
       ]
     },