`om ci`: Support for `use`ing & `push`ing to cachix

[srid]

... in addition to 'whitelist pushing' (via pins).

Basically replace this entire module via om.ci.default.<..> configuration, and have om ci take care of the rest as part its build.

https://github.com/juspay/omnix/blob/2aa1f48783b9757e87e9c54f5f250bb56384f066/nix/modules/cache-pins.nix#L41-L48

• The design should be cache agnostic, while supporting cachix to begin with (in future, we can support attic and the like).

This will obviate the following two steps in CI, in addition to the aforementioned flake-module:

https://github.com/juspay/omnix/blob/2aa1f48783b9757e87e9c54f5f250bb56384f066/.github/workflows/ci.yaml#L19-L24

https://github.com/juspay/omnix/blob/2aa1f48783b9757e87e9c54f5f250bb56384f066/.github/workflows/ci.yaml#L70-L72

[srid]

Perhaps this should be decoupled and built on top of #198

[srid]

I tried to implement it only to realize that it is not that simple. Also, we don't want to complicate omnix, when the user could simply use unix tools to achieve this. For example,

$ om ci run . | rg -e "-omnix-cli-\d+\.\d+\.\d+$" -e "-omnix-mdbook-site$"
/nix/store/ppnag6lacdlg8q3vz3avv6da942ffgqw-omnix-mdbook-site
/nix/store/whdiim2js86xq14qdyk4mxfsdgni6cnh-omnix-cli-0.1.0

$ om ci run . | rg -e "-omnix-cli-\d+\.\d+\.\d+$" -e "-omnix-mdbook-site$" | xargs cachix push om

But to implement pinning (which requires naming each path to be pinned), we'll do https://github.com/juspay/cachix-push/issues/6 - after which the user can simply run the following in sequence:

$ om ci run .
$ nix run .#cachix-push

Keeping cache pushes off CI is good for another reason -- it makes no sense to do that when doing local CI.

[srid]

https://github.com/juspay/cachix-push/pull/7

Will document this as well.