I've just done a fresh install of 1.8.13 and then added https://raw.githubusercontent.com/damellis/attiny/ide-1.6.x-boards-manager/package_damellis_attiny_index.json to the boards manager.
When I try to compile and upload a simple test example I get the error:
An error occurred while uploading the sketch
avrdude: can't open config file "C:\Users\robert\AppData\Local\Arduino15\packages\attiny\hardware\avr\1.0.2/avrdude.conf": No such file or directory
avrdude: error reading system wide configuration file "C:\Users\robert\AppData\Local\Arduino15\packages\attiny\hardware\avr\1.0.2/avrdude.conf"
I notice there's no avrdude.conf in the github repo. Should there be? There's a conf file in the MightyCore/hardware/avr/2.1.3 directory and there's actually an entry for ATtiny85 in "C:\Users\xxx\AppData\Local\Arduino15\packages\attiny\hardware\avr\1.0.2/avrdude.conf" for the nightly build.
What do I need to do to use the ATtiny with the IDE?