[CI/Build][Bugfix] fix qutlass cmake error when set QUTLASS_SRC_DIR (#26773)

Signed-off-by: izhuhaoran <izhuhaoran@qq.com>
Co-authored-by: Michael Goin <mgoin64@gmail.com>

cmake/external_projects/qutlass.cmake

@@ -22,10 +22,10 @@ else()
     CONFIGURE_COMMAND ""
     BUILD_COMMAND ""
   )
-  FetchContent_Populate(qutlass)
-  set(qutlass_SOURCE_DIR "${qutlass_SOURCE_DIR}")
 endif()
 
+FetchContent_Populate(qutlass)
+
 if(NOT qutlass_SOURCE_DIR)
   message(FATAL_ERROR "[QUTLASS] source directory could not be resolved.")
 endif()