Here's a completely rewritten "Arduino as ISP" sketch that can program the ATtiny85, in situ, on the Digispark board via its 6- and 3- pin connectors. It uses an Arduino Uno R3 as the programmer, and ...

This is a fork of the code and examples from "Make: AVR Programming" that I've addapted to work with the Arduino Uno R3 (I'm using the SMD variant, but it shouldn't matter which UNO R3 you're using).