Nix how to use alternative store path for CI caching

Question

Many CI providers give you a directory whose contents are retained across builds and you can use that as a cache. Everything that is stored elsewhere is lost. This means that any artefacts that are created during a nix-build that are placed in the nix store (/nix/store) are lost. I'm trying to figure out how to convince nix to prefer that other cache directory over the global /nix/store. However the documentation is a bit lacking.

What I've tried so far:

• Add file:///the/path to substituters and then nix copy --to that path. However I discovered that nix only creates some metadata files in that directory and copies the actual derivation into /nix/store. That's not what I want.
• Use local?root=/the/path instead of the file:// url (btw, this syntax is not documented anywhere, I only found it in a single github issue!). That made nix copy the whole derivation to that folder, but I couldn't figure out how to convince nix-build to actually consult that store during build.

Answer 1

Would it be possible to use something along the lines of nix run --store ~/my-nix nixpkgs.hello -c hello --greeting 'Hi everybody!'? The installation guide points to uses of --store for such a use case, as well as some sections in the manual: 1, 2.

An example of this can be found in nix run's tests.

There are also the environment variables NIX_STORE_DIR which might be of use? It's documented in nix-shell --help.

There's also several issues in the Nix repo, here's an interesting discussion.