Have you considered bundling that env into your CI image? I have a CI/CD process that will build and update an image with the environment pre-loaded. It doesn't save you the data transfer, but it likely provides a better warm cache.
Is there an easier way to do it?
Also, wouldn't it be pretty similar performance-wise to caching the /nix path on a generic Nix image? I've tried that, but it has problems because the Nix path ends up being a few GB, so that ends up adding a long time to the build as well.
- A bare image with just Nix can pull the deps from a cache.
- A pre-built image contains the deps from the latest push to default branch and nothing else.
I do not maintain a separate repo for this. Part of my CI is building and caching the paths. And then CD is building and pushing the image, which the CI has verified as working. This requires using a mutable tag, but you could also have the CD process auto-update the tag if this is not an option.
You're correct that this doesn't save you from having to get the same amount of data to the same place, but the image repo can usually be cached more advantageously for whatever is pulling images, so it should be considerably faster overall.