Update Windows build to use VULKAN_SDK rather than VK_SDK_PATH

gyp/gpu.gyp

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 # GYP for building gpu
 {
   'target_defaults': {
     'conditions': [
       ['skia_os != "win"', {
         'sources/': [ ['exclude', '_win.(h|cpp)$'],
@@ skipping 166 matching lines @@
           'link_settings': {
             'libraries': [ '<(vulkan_lib_name)', ],
           },
           'dependencies': [
             'shaderc.gyp:shaderc_combined',
           ],
           'conditions': [
             [ 'skia_os == "win"', {
              'variables': {
                 'vulkan_lib_name': '-lvulkan-1',
-                'vulkan_sdk_path' : '<!(echo %VK_SDK_PATH%)',
+                'vulkan_sdk_path' : '<!(echo %VULKAN_SDK%)',
               },
               'include_dirs': [
                 '<(vulkan_sdk_path)/Include',
               ],
               'direct_dependent_settings': {
                 'include_dirs': [
                   '<(vulkan_sdk_path)/Include',
                 ],
               },
               'link_settings': {
@@ skipping 54 matching lines @@
           ],
         }, {
           'sources!': [
             '<@(skgpu_vk_sources)',
           ],
         }],
       ],
     },
   ],
 }