Emscripten: Restore support for indirect use of emsdk toolchain file

Extend commit 644ad91e6a (Emscripten: Restore support for emsdk
toolchain file and platform module, 2025-09-17) to support using a
third-party `CMAKE_TOOLCHAIN_FILE` that includes the emsdk-provided
toolchain file.

Issue: #27232
1 file changed