[libc++/libc++abi] Use Python3_EXECUTABLE consistently to run utilities

GitOrigin-RevId: 1b2fa6e46e4e6961254ff9410b0a9eedbbe22e40
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 7093cef..ec88a1e 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -285,7 +285,7 @@
   # Merge the libc++abi.a and libunwind.a into one.
   if(LIBCXXABI_USE_LLVM_UNWINDER AND LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_STATIC_LIBRARY)
     add_custom_command(TARGET cxxabi_static POST_BUILD
-      COMMAND ${PYTHON_EXECUTABLE} ${LIBCXXABI_LIBCXX_PATH}/utils/merge_archives.py
+      COMMAND ${Python3_EXECUTABLE} ${LIBCXXABI_LIBCXX_PATH}/utils/merge_archives.py
       ARGS
         -o "$<TARGET_LINKER_FILE:cxxabi_static>"
         --ar "${CMAKE_AR}"