[infra] Switch cross-arm64 builder to use a single shard

Triggering on arm64 fails because some kitchen CIPD packages
are unavailable on arm64.

TBR=whesse@google.com

Change-Id: I26b05d000dde712ab1b1d55a33be1e4a02f22247
Reviewed-on: https://dart-review.googlesource.com/c/84424
Reviewed-by: Alexander Thomas <athom@google.com>
diff --git a/tools/bots/test_matrix.json b/tools/bots/test_matrix.json
index e6bde5d..afd024e 100644
--- a/tools/bots/test_matrix.json
+++ b/tools/bots/test_matrix.json
@@ -119,7 +119,7 @@
       "out/ReleaseIA32/",
       "out/ReleaseSIMARM/",
       "out/ReleaseSIMARM64/",
-      "out/ReleaseARM64/",
+      "out/ReleaseXARM64/",
       "out/ReleaseSIMDBC64/",
       "out/ReleaseX64/",
       "xcodebuild/DebugIA32/",
@@ -680,23 +680,10 @@
           "arguments": ["runtime_kernel"]
         },
         {
-          "name": "trigger target arm builder",
-          "fileset": "vm-kernel",
-          "trigger": ["target-vm-linux-release-arm64"]
-        }
-      ]
-    },
-    {
-      "builders": [
-        "target-vm-linux-release-arm64"
-      ],
-      "meta": {
-        "description": "This configuration is for the target arm builders."
-      },
-      "steps": [
-        {
           "name": "vm tests",
-          "arguments": ["-ndartk-${system}-${mode}-${arch}"]
+          "arguments": ["-ndartk-${system}-${mode}-${arch}"],
+          "fileset": "vm-kernel",
+          "shards": 1
         }
       ]
     },