[infra] Balance the number shards on the AOT builder with co19_2 tests

The regular test shards run in about half the time. Therefore, we can
use less shards for overall efficiency without sacrificing cycle time.

TBR=asiva@google.com
Change-Id: Ib8b58a03f9c40d0e8f288cde6e148a54fc1ed1e5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138300
Reviewed-by: Alexander Thomas <athom@google.com>

Author: athomas

tools/bots/test_matrix.json

@@ -1335,9 +1335,9 @@
             "-ndartkp-${system}-${mode}-${arch}"
           ],
           "fileset": "vm-kernel",
-          "shards": 10
+          "shards": 6
         },
-	{
+        {
           "name": "vm co19_2 tests",
           "arguments": [
             "-ndartkp-${system}-${mode}-${arch}",