Update Windows build to use VULKAN_SDK rather than VK_SDK_PATH

infra/bots/recipes/swarm_compile.expected/Build-Win-MSVC-x86_64-Release-Vulkan.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[CUSTOM_C:\\_B_WORK]",
       "511"
     ],
     "name": "makedirs checkout_path",
@@ skipping 82 matching lines @@
       "most"
     ],
     "cwd": "[CUSTOM_C:\\_B_WORK]\\skia",
     "env": {
       "BUILDTYPE": "Release_x64",
       "CHROME_HEADLESS": "1",
       "CHROME_PATH": "[SLAVE_BUILD]\\src",
       "GYP_DEFINES": "qt_sdk=C:/Qt/4.8.5/ skia_arch_type=x86_64 skia_vulkan=1 skia_vulkan_debug_layers=0 skia_warnings_as_errors=1 skia_win_debuggers_path=c:/DbgHelp skia_win_ltcg=0",
       "PATH": "%(PATH)s;RECIPE_PACKAGE_REPO[depot_tools];RECIPE_PACKAGE_REPO[depot_tools]",
       "SKIA_OUT": "[CUSTOM_C:\\_B_WORK]\\skia\\out\\Build-Win-MSVC-x86_64-Release-Vulkan",
-      "VK_SDK_PATH": "[SLAVE_BUILD]\\win_vulkan_sdk"
+      "VULKAN_SDK": "[SLAVE_BUILD]\\win_vulkan_sdk"
     },
     "name": "build most"
   },
   {
     "cmd": [
       "python",
       "-u",
       "import errno\nimport glob\nimport os\nimport shutil\nimport sys\n\nsrc = sys.argv[1]\ndst = sys.argv[2]\nbuild_products_whitelist = ['dm', 'dm.exe', 'get_images_from_skps', 'get_images_from_skps.exe', 'nanobench', 'nanobench.exe', '*.so', '*.dll', '*.dylib', 'skia_launcher', 'lib/*.so', 'iOSShell.app', 'iOSShell.ipa', 'visualbench', 'visualbench.exe', 'vulkan-1.dll']\n\ntry:\n  os.makedirs(dst)\nexcept OSError as e:\n  if e.errno != errno.EEXIST:\n    raise\n\nfor pattern in build_products_whitelist:\n  path = os.path.join(src, pattern)\n  for f in glob.glob(path):\n    dst_path = os.path.join(dst, os.path.relpath(f, src))\n    if not os.path.isdir(os.path.dirname(dst_path)):\n      os.makedirs(os.path.dirname(dst_path))\n    print 'Copying build product %s to %s' % (f, dst_path)\n    shutil.move(f, dst_path)\n",
       "[CUSTOM_C:\\_B_WORK]\\skia\\out\\Build-Win-MSVC-x86_64-Release-Vulkan\\Release_x64",
       "[CUSTOM_[SWARM_OUT_DIR]]\\out\\Release_x64"
@@ skipping 81 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@    pass@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]