[ci] use nix caching for actions cache

[VTimofeenko]

No description provided.

[musjj]

Would love to have this too.

There's instructions here: https://nix.dev/tutorials/nixos/continuous-integration-github-actions. Shouldn't be hard to set it up.

Also, it's free for open-source projects (up to 5GB).

[VTimofeenko]

I was actually thinking about magic-nix-cache as cachix requires specifying secrets on the repo level and ownership of one more system. I'd rather keep this project self-contained on github.

Currently the build action is configured to run <=> the package definition changes, and most of the development in this repo is done on modules, so there is little benefit to be had in this for now.

It would probably be more relevant when qemu-based tests are done in #16

[musjj]

I don't think end-users can use the magic-nix-cache as a binary cache. It's only meant to be used to speed up the CI.
I thought the issue was about providing a binary cache so that users don't have to build the program locally.

[VTimofeenko]

Yeah, I don't think github cache can be made public. End-user facing cache would be #29

[musjj]

Oh thanks, didn't notice that issue.
Btw, does this concern:

cachix requires specifying secrets on the repo level and ownership of one more system. I'd rather keep this project self-contained on github.

also applies to the end-user facing cache? Because I don't think it'd be possible to provide a nix binary cache that is self-contained on GitHub.