This fixes a bug in SystemInfo_vulkan where we were using the
System search path for the Vulkan loader when we prefer using the
custom ANGLE loader.

Bug: chromium:1219969
Change-Id: Iedf0fd11fe9ed8cc020b445ea9e12a7936937361
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2988791Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>