Add drone_dimensions as top level target property. (flutter#42064)

This is to make the configs easier to use for builders running from a drone.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

.ci.yaml

@@ -262,17 +262,17 @@ targets:
     properties:
       release_build: "true"
       config_name: linux_fuchsia
-      drone_dimensions: >
-        ["os=Linux"]
+    drone_dimensions:
+      - os=Linux
 
   - name: Linux linux_arm_host_engine
     recipe: engine_v2/engine_v2
     timeout: 60
     properties:
       release_build: "true"
       config_name: linux_arm_host_engine
-      drone_dimensions: >
-        ["os=Linux"]
+    drone_dimensions:
+      - os=Linux
 
   - name: Linux linux_host_engine
     recipe: engine_v2/engine_v2
@@ -289,26 +289,26 @@ targets:
     properties:
       release_build: "true"
       config_name: linux_host_desktop_engine
-      drone_dimensions: >
-        ["os=Linux"]
+    drone_dimensions:
+      - os=Linux
 
   - name: Linux linux_android_aot_engine
     recipe: engine_v2/engine_v2
     timeout: 60
     properties:
       release_build: "true"
       config_name: linux_android_aot_engine
-      drone_dimensions: >
-        ["os=Linux"]
+    drone_dimensions:
+      - os=Linux
 
   - name: Linux linux_android_debug_engine
     recipe: engine_v2/engine_v2
     timeout: 60
     properties:
       release_build: "true"
       config_name: linux_android_debug_engine
-      drone_dimensions: >
-        ["os=Linux"]
+    drone_dimensions:
+      - os=Linux
 
   - name: Linux linux_license
     recipe: engine_v2/builder
@@ -323,8 +323,8 @@ targets:
     properties:
       release_build: "true"
       config_name: linux_web_engine
-      drone_dimensions: >
-        ["os=Linux"]
+    drone_dimensions:
+      - os=Linux
 
   - name: Linux linux_unopt
     recipe: engine_v2/engine_v2
@@ -389,8 +389,8 @@ targets:
     properties:
       release_build: "true"
       config_name: mac_android_aot_engine
-      drone_dimensions: >
-        ["os=Linux"]
+    drone_dimensions:
+      - os=Linux
 
   - name: Mac mac_clang_tidy
     recipe: engine_v2/engine_v2
@@ -424,8 +424,8 @@ targets:
         ]
       $flutter/osx_sdk : >-
         { "sdk_version": "14e222b" }
-      drone_dimensions: >
-        ["os=Mac-12"]
+    drone_dimensions:
+      - os=Mac-12
 
   - name: Linux mac_unopt
     recipe: engine_v2/engine_v2
@@ -511,8 +511,8 @@ targets:
         [
           {"dependency": "jazzy", "version": "0.14.1"}
         ]
-      drone_dimensions: >
-        ["os=Mac-12"]
+    drone_dimensions:
+      - os=Mac-12
 
   - name: Mac impeller-cmake-example
     bringup: true
@@ -547,17 +547,17 @@ targets:
     properties:
       release_build: "true"
       config_name: windows_android_aot_engine
-      drone_dimensions: >
-        ["os=Windows"]
+    drone_dimensions:
+      - os=Windows
 
   - name: Windows windows_host_engine
     recipe: engine_v2/engine_v2
     timeout: 60
     properties:
       release_build: "true"
       config_name: windows_host_engine
-      drone_dimensions: >
-        ["os=Windows"]
+    drone_dimensions:
+      - os=Windows
 
   - name: Windows windows_arm_host_engine
     recipe: engine_v2/engine_v2
@@ -568,8 +568,8 @@ targets:
     properties:
       release_build: "true"
       config_name: windows_arm_host_engine
-      drone_dimensions: >
-        ["os=Windows"]
+    drone_dimensions:
+      - os=Windows
 
   - name: Windows windows_unopt
     recipe: engine_v2/builder