xchange-94: Use VK_ prefix for environment vars

Use the Vulkan namespace (VK_ prefix) on debug environment
variables used in the loader and the Intel sample driver.
This resolves LunarXchange bug #94.
https://vulkan.lunarg.com/app/issues/55b76baa7ef24d0001000072
diff --git a/loader/loader.c b/loader/loader.c
index 8fd4b10..c3e28fb 100644
--- a/loader/loader.c
+++ b/loader/loader.c
@@ -987,7 +987,7 @@
     g_loader_debug = 0;
 
     /* parse comma-separated debug options */
-    env = getenv("LOADER_DEBUG");
+    env = getenv("VK_LOADER_DEBUG");
     while (env) {
         const char *p = strchr(env, ',');
         size_t len;