RFC: Warn if caching is not working / Make it easier to diagnose misconfigured caches on installation

[madeline-os]

The practicality and user experience of obelisk depends on robust binary caching. This is true even before obelisk has been installed: being forced to compile everything from scratch is unacceptable. This is also something that can easily be overlooked when it goes wrong.

If it is possible, it would be desirable to warn if obelisk is being compiled from source on installation.

[augyg]

Would obelisk have permissions to just parse a nix.conf or configuration.nix? That seems like an easy way to check this

I suppose for configuration.nix you might want to call nix-instantiate on it.

I'm curious how much control we'd have over that process given that it's nix-env running the installation