arduino-tiny: change TIMER_TO_USE_FOR_MILLIS from sketch
Looking at the Arduino-Tiny core in the file core_build_options.h the following code appears depending on the processor type:

#define TIMER_TO_USE_FOR_MILLIS 1


That header is then included in all the other parts of the core code so I don't see any possibility to override it directly from your sketch.

I'm not an Arduino user so don't really know recommendations on changing the core libraries but maybe you could change the code in core_build_options.h to get up and running. I noticed it occurs a few times depending on the processor type so make sure you're changing the correct piece of code.

When / if you upgrade the core files you'll lose that change. To make sure you don't forget to update it in the future you could something like the following in your sketch to throw a compiler error if it's not defined as you expect:

#include "correct_path\core_build_options.h" // Not sure if this will be required
#if TIMER_TO_USE_FOR_MILLIS != 0
    #error TIMER_TO_USE_FOR_MILLIS is wrong value
#endif


Depending on the order things are compiled in you also maybe able to change the header file to use something like this that will only set the value if it was previously undefined so the operation will remain the same for other sketches:

#ifndef TIMER_TO_USE_FOR_MILLIS
    #define TIMER_TO_USE_FOR_MILLIS 1
#endif