Arm backend: Fix set of VULKAN_SDK (#23163)

`VULKAN_SDK` was being added using the generic path-list helper,
 which appends `:` to the value. When the variable was previously
unset, this produced a path like `/path/to/vulkan-sdk:`, which is not a
valid directory and caused the VGF environment check to fail.

Use the scalar environment helper instead, so `VULKAN_SDK` is exported
as the exact SDK path without the trailing `:`.


cc @SS-JIA @manuelcandales @digantdesai @cbilgin @freddan80 @per @zingo
@oscarandersson8218 @mansnils @Sebastian-Larsson @robell @rascani

Signed-off-by: Elena Zhelezina <elena.zhelezina@arm.com>

Author: wwwind

backends/arm/scripts/vulkan_utils.sh

@@ -180,5 +180,5 @@ function setup_path_vulkan() {
     else
         prepend_env_in_setup_path LD_LIBRARY_PATH "${vulkan_sdk_arch_root}/lib"
     fi
-    prepend_env_in_setup_path VULKAN_SDK "${vulkan_sdk_arch_root}"
+    set_env_in_setup_path VULKAN_SDK "${vulkan_sdk_arch_root}"
 }