Restore the ability to not build vulkan when VULKAN_SDK is set

After https://skia.googlesource.com/skia/+/686bb21f3ed19218a85e94798fe50e240e3fac08 skia_use_vulkan=false stopped working when VULKAN_SDK is set in the environment.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4038

Change-Id: I3707c930d041556d1727f43c48eba2cad73349eb
Reviewed-on: https://skia-review.googlesource.com/4038
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
1 file changed