Set vulkan_loader_shared to false on Mac.

This is the same configuation that ANGLE uses. Without this, Release
builds of Dawn on Mac can't find libvulkan.dylib.

Bug: angleproject:6981

Change-Id: Ib5eddd0f694d1b960e16341578427fa4f0a009c5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/79560
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Stephen White <senorblanco@chromium.org>
diff --git a/build_overrides/vulkan_loader.gni b/build_overrides/vulkan_loader.gni
index ac6a61d..6f6eaf0 100644
--- a/build_overrides/vulkan_loader.gni
+++ b/build_overrides/vulkan_loader.gni
@@ -14,4 +14,4 @@
 
 import("//build_overrides/vulkan_common.gni")
 
-vulkan_loader_shared = true
+vulkan_loader_shared = !is_mac