[cmake] Tweak warning in extend_path helper function

There was one more reference the word "install" I forgot to remove.

Follow-up to bde561c481 /
https://reviews.llvm.org/D115746

cmake/Modules/ExtendPath.cmake

@@ -10,7 +10,7 @@ function(extend_path joined_path base_path current_segment)
   elseif("${base_path}" STREQUAL "")
     set(temp_path "${current_segment}")
   elseif(IS_ABSOLUTE "${current_segment}")
-    message(WARNING "Since \"${current_segment}\" is absolute, it overrides install path: \"${base_path}\".")
+    message(WARNING "Since \"${current_segment}\" is absolute, it overrides base path: \"${base_path}\".")
     set(temp_path "${current_segment}")
   else()
     set(temp_path "${base_path}/${current_segment}")