The tricky bit for this was getting a unity license into the secrets. There's some docs on this here: https://game.ci/docs/github/activation#personal-license
I created a new unity account explicitly for this usage so i didn't need to use my own.
--
Just noting that I abandoned using this at all b/c it ran so slow ~40 minutes. This could probably be mitigated if you pay for a bigger CI machine, but I settled for building and deploying locally via butler (itch's cli tool)