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

Change-Id: I720be4bdb27fe41c76f4d485d9a6d9ce5b7a5357
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/404700
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

tools/bots/test_matrix.json

diff --git a/tools/bots/test_matrix.json b/tools/bots/test_matrix.json
index 5664b97..522a9f4 100644
--- a/tools/bots/test_matrix.json
+++ b/tools/bots/test_matrix.json
@@ -1772,7 +1772,24 @@
       },
       "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/bin:dart - divided to avoid timeout",
+          "script": "tools/build.py",
+          "arguments": [
+            "--no-clang",
+            "runtime/bin:dart",
+            "runtime/bin:run_vm_tests"
+          ]
+        },
+        {
+          "name": "build dart - step 3 - runtime - divided to avoid timeout",
           "script": "tools/build.py",
           "arguments": [
             "--no-clang",