Cache push is special case of non-Nix actions #97
Description
Presently, vira hardcodes support for cachix and attic (pushing to remote cache).
But this is a special case of a general "actions" that the CI can do aside from doing Nix builds. For example, building and pushing container images to ghcr.io (example) - and that is no different, in essence, to pushing to Nix cache. Both require certain secrets to be available in CI as well.
How do we deal with this? Users may want to add their own action, if Vira is to be used as exclusive CI. Not to mention as #96