[UR] Ensure UNIFIED_RUNTIME_SOURCE_DIR is set

Author: kbenzie

sycl/cmake/modules/FetchUnifiedRuntime.cmake

@@ -75,7 +75,10 @@ cmake_path(NORMAL_PATH UR_INTREE_SOURCE_DIR OUTPUT_VARIABLE UR_INTREE_SOURCE_DIR
 
 if(IS_DIRECTORY "${UR_INTREE_SOURCE_DIR}")
   set(UR_INTREE_BINARY_DIR ${LLVM_BINARY_DIR}/unified-runtime)
-  add_subdirectory(${UR_INTREE_SOURCE_DIR} ${UR_INTREE_BINARY_DIR})
+  set(UNIFIED_RUNTIME_SOURCE_DIR
+    "${UR_INTREE_SOURCE_DIR}" CACHE PATH
+    "Path to Unified Runtime Headers" FORCE)
+  add_subdirectory(${UNIFIED_RUNTIME_SOURCE_DIR} ${UR_INTREE_BINARY_DIR})
 elseif(SYCL_UR_USE_FETCH_CONTENT)
   include(FetchContent)