[infra] Split build steps for aot-win-{debug,release}-arm64 to avoid timeouts.

Number of targets per splits are 982/800/1829.

Example of failing build https://ci.chromium.org/ui/p/dart/builders/ci.sandbox/vm-aot-win-debug-arm64/1084

Change-Id: I59ee16768c30ad4b5940cb745d92dcd874a0bbf2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/406040
Reviewed-by: Alexander Thomas <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

tools/bots/test_matrix.json

@@ -1823,12 +1823,26 @@
       },
       "steps": [
         {
-          "name": "build dart",
+          "name": "build dart - step 1 - gen-snapshot - divided to avoid timeout",
+          "script": "tools/build.py",
+          "arguments": [
+            "--no-clang",
+            "gen_snapshot"
+          ]
+        },
+        {
+          "name": "build dart - step 2 - runtime - divided to avoid timeout",
+          "script": "tools/build.py",
+          "arguments": [
+            "--no-clang",
+            "dartaotruntime"
+          ]
+        },
+        {
+          "name": "build dart - step 3 - runtime/bin:dart - divided to avoid timeout",
           "script": "tools/build.py",
           "arguments": [
             "--no-clang",
-            "dartaotruntime",
-            "gen_snapshot",
             "runtime"
           ]
         },