feat: build nix flake with action

[LajnaLegenden]

This pull request introduces a new GitHub Actions workflow to automate the build process for the NixOS flake. The workflow is triggered on pushes to the main branch and pull requests, and it sets up a Nix cache, installs Nix, and builds the flake.

New GitHub Actions workflow for NixOS flake:

• .github/workflows/build.yaml: Added a new workflow named "NIXOS build flake" to automate the build process. It includes steps to check out the repository, set up a Nix cache, install Nix using the cachix/install-nix-action, and build the flake using nix build. The workflow is triggered on push events to the main branch and on pull requests. :)

[LajnaLegenden]

Had an old action around that works fine for this if you'd like to have a sanity check so I prs atleast builds before merging