use system' spriv-tools not the bundled one

Index: glslc/test/CMakeLists.txt
--- glslc/test/CMakeLists.txt.orig
+++ glslc/test/CMakeLists.txt
@@ -23,6 +23,6 @@ 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()
