[UR] Update how UR_OPENCL_ICD_LOADER_LIBRARY is set (intel#11997)

In oneapi-src/unified-runtime#993 the type of
the `UR_OPENCL_ICD_LOADER_LIBRARY` CMake variable will change to a cache
variable, as such the usage of this variable also needs to up updated.

sycl/plugins/unified_runtime/CMakeLists.txt

@@ -30,7 +30,8 @@ if("hip" IN_LIST SYCL_ENABLE_PLUGINS)
 endif()
 if("opencl" IN_LIST SYCL_ENABLE_PLUGINS)
   set(UR_BUILD_ADAPTER_OPENCL ON)
-  set(UR_OPENCL_ICD_LOADER_LIBRARY OpenCL-ICD)
+  set(UR_OPENCL_ICD_LOADER_LIBRARY OpenCL-ICD CACHE FILEPATH
+    "Path of the OpenCL ICD Loader library" FORCE)
 endif()
 if("native_cpu" IN_LIST SYCL_ENABLE_PLUGINS)
   set(UR_BUILD_ADAPTER_NATIVE_CPU ON)