It depends on what you use. If you install Nim with _choosenim_ you will need to cache `~/.choosenim` directory. If you use any dependencies installed with _nimble_ , you will need to cache `~/.nimble` directory.
BTW, GitHub Nim action: <https://github.com/jiro4989/setup-nim-action> has support for caching plus some examples how to use it. That's probably the easiest way to set Nim with GitHub Actions.
