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I'm trying to build polysemy in CI on https://builds.sr.ht/~thematten/job/344671, but (using nix-env -iA) it seems to build everything from source, which is not what I see on my devices - am I missing something or is it ignoring nixpkgs cache?
You need to put your cache somewhere and call it again when building that. For example, you use cachix, you build polysemy in your machine and push the build into the cachix. Then, in your CI, you use your cachix's credentials and then you build the project. It will automatically download from the cache.
this is about dependencies that should be in cache.nixos.org