Implement an automated workflow to integrate and deploy NixOS configurations

[btlogy]

Scope

Part of #4161 - Move off Trac - Execution - Forgejo

Currently, the Nix code in this repository is not automatically built, nor deployed on the targeted system.
This is error prone and slowing down changes.
We could implement GHA workflow to continuously integrate and deploy the Nix configurations defined in this repository.

Value

• Contributors could have their pull request automatically checked
• Maintainers would not have to manually deploy all changes

Requirements

• Automatic checks verifying and building the flake on pull request
• Automatic deployment of the nixosConfiguration('s) whenever a pull request is merged into the main branch

Additional information