dependencies: support old vulkan SDK version

In old versions of Vulkan SDK, VK_SDK_PATH environment variable was used instead
of VULKAN_SDK.

This patch check both environment variables is fallback mode.

mesonbuild/dependencies/ui.py

@@ -189,7 +189,7 @@ def __init__(self, name: str, environment: 'Environment', kwargs: T.Dict[str, T.
         super().__init__(name, environment, kwargs, language=language)
 
         try:
-            self.vulkan_sdk = os.environ['VULKAN_SDK']
+            self.vulkan_sdk = os.getenv('VULKAN_SDK', os.environ['VK_SDK_PATH'])
             if not os.path.isabs(self.vulkan_sdk):
                 raise DependencyException('VULKAN_SDK must be an absolute path.')
         except KeyError: