I have a very small application that requires some seriously big nixpkgs rebuilds (they take about half an hour on my machine). Is there any way to cache /nix between builds so that I don’t have to rebuild? Manual says that the cached directory needs to be writable by travis user, and /nix is most definetely not.
You probably want to use https://cachix.org
As @kreisys says, integrating this with cachix is pretty neat. See https://github.com/entropia/tip-toi-reveng/blob/2d35417e37788c629500b7c82cd82e2ff9c1e89b/.travis.yml for an example.
In case anyone else lands here, the nix wiki describes how to set up nix on travis with a cross-build cache
https://nixos.wiki/wiki/Nix_on_Travis, which can be used in addition to cachix.