Prepare for std-action-custom-runner

[blaggacao]

#13 lays the groundwork for the possibility to consume a s3-like cache.

This issue should modify the evaluator such as that if it detects an initial presence of a nix store on the runner, don't engage in any GH caching activity.

[nrdxp]

Started implementing a NixOS configuration that folks can use on the official NixOS AMIs. So that the steps for setting up your own runner would be as simple as:

• spin up an instance based on the latest NixOS ami
• from the instance run nixos-rebuild --flake divnix/std-action-runner#$system

And that's basically it on the infra side, configuring github to use it can be done in the web ui. Here is the repo:
https://github.com/divnix/std-action-runner