The nixpkgs is now assumed from the outside. This allows for the config
to have more control with the nixpkgs to be imported with the
home-manager configuration.
Now we can finally easily declare the nixpkgs channel for individual
images. This is nice once I've expanded the images for other purposes
such as physical hosts (for homelabbing).
Unfortunately, this is where I realized that stable channels are not
exactly "Debian-stable" and the best way to handle security is to have
an update cadence that is relatively quick.
Additionally, I have also made a declarative version of the project
cluster with a neat little file at `images.toml`. This is way better for
individually referring to hosts from the host directory compared to the
old way.
