[et] Add ios_debug_unopt build config.

ci/builders/local_engine.json

@@ -32,6 +32,39 @@
         }
       }
     },
+    {
+      "cas_archive": false,
+      "drone_dimensions": [
+        "os=Mac-13",
+        "device_type=none"
+      ],
+      "gclient_variables": {
+        "download_android_deps": false,
+        "use_rbe": true
+      },
+      "gn": [
+        "--ios",
+        "--runtime-mode",
+        "debug",
+        "--unoptimized",
+        "--no-stripped",
+        "--no-lto",
+        "--xcode-symlinks",
+        "--rbe",
+        "--no-goma"
+      ],
+      "name": "macos/ios_debug_unopt",
+      "description": "Builds an unoptimized debug mode engine that targets iOS from a macOS host.",
+      "ninja": {
+        "config": "ios_debug_unopt",
+        "targets": []
+      },
+      "properties": {
+        "$flutter/osx_sdk": {
+          "sdk_version": "15a240d"
+        }
+      }
+    },
     {
       "cas_archive": false,
       "drone_dimensions": [