We were previously trying to define this macro through an env var, which actually only affects the make invocation that extconf.rb uses to build the grpc core library on non-windows platforms.

Note that all of the other CPPFLAGS set through env vars are targeting the grpc core library, rather than the ruby C-extension code, so it worked. But this new macro for windows ucrt is targeting the ruby C-extension code, so it looks like it needs to be set via $CFLAGS.

Should fix #30933

cc @johnnyshields