must set DAWN_EMSCRIPTEN_TOOLCHAIN for build too

Author: MichealReed

cmake/dawn.cmake

@@ -6,6 +6,7 @@ set(DAWN_BUILD_DIR "${DAWN_DIR}/build" CACHE INTERNAL "")
 if(EMSCRIPTEN)
     set(EM_SDK_DIR $ENV{EMSDK} CACHE INTERNAL "")
     set(DAWN_BUILD_DIR "${DAWN_DIR}/build_web" CACHE INTERNAL "")
+    set(DAWN_EMSCRIPTEN_TOOLCHAIN ${EM_SDK_DIR}/upstream/emscripten CACHE INTERNAL "" FORCE)
 endif()
 
 # Enable find for no dawn rebuilds with flutter run