Default vulkan sdk to VULKAN_SDK env var in GN

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4032

Change-Id: I48767cbbe7e0cd306e448bdc983c853f66121e1b
Reviewed-on: https://skia-review.googlesource.com/4032
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Greg Daniel <egdaniel@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 30f58fe..f6e4f0c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -28,7 +28,7 @@
   skia_enable_gpu = true
   skia_enable_tools = is_skia_standalone
   skia_enable_vulkan_debug_layers = is_skia_standalone && is_debug
-  skia_vulkan_sdk = ""
+  skia_vulkan_sdk = getenv("VULKAN_SDK")
 }
 
 skia_use_vulkan = skia_use_vulkan || skia_vulkan_sdk != ""
@@ -123,8 +123,8 @@
         include_dirs += [ "$skia_vulkan_sdk/Include/" ]
         lib_dirs += [ "$skia_vulkan_sdk/Bin" ]
       } else {
-        include_dirs += [ "$skia_vulkan_sdk/x86_64/include/" ]
-        lib_dirs += [ "$skia_vulkan_sdk/x86_64/lib/" ]
+        include_dirs += [ "$skia_vulkan_sdk/include/" ]
+        lib_dirs += [ "$skia_vulkan_sdk/lib/" ]
       }
     }
     if (is_win) {