use system' spriv-tools not the bundled one

Index: glslc/test/CMakeLists.txt
--- glslc/test/CMakeLists.txt.orig
+++ glslc/test/CMakeLists.txt
@@ -27,7 +27,7 @@ if(${SHADERC_ENABLE_TESTS})
     add_test(NAME glslc_tests
       COMMAND ${Python_EXECUTABLE}
       ${CMAKE_CURRENT_SOURCE_DIR}/glslc_test_framework.py
-      $<TARGET_FILE:glslc_exe> $<TARGET_FILE:spirv-dis>
+      $<TARGET_FILE:glslc_exe> spirv-dis
       --test-dir ${CMAKE_CURRENT_SOURCE_DIR})
   endif()
 
