ci: switch images from Docker Hub to GitHub Packages

Docker changed their mind and are asking us to pay to keep the org on
Docker Hub.

.github/workflows/nix.yml

@@ -8,6 +8,10 @@ on:
   schedule:
     # Run once per day
     - cron: '0 0 * * *'
+
+env:
+  CI_REGISTRY: ghcr.io
+
 jobs:
   build:
     strategy:
@@ -18,11 +22,23 @@ jobs:
           - nixos-unstable
           - nixos-22.05
           - nixos-22.11
+
     runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+      packages: write
+
     steps:
       - uses: actions/checkout@v3
+      - name: Log in to the Container registry
+        uses: docker/login-action@v2.1.0
+        with:
+          registry: ${{ env.CI_REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
       - uses: cachix/install-nix-action@v20
       - run: nix-shell --run ./ci.sh
         env:
-          CI_REGISTRY_AUTH: '${{ secrets.REGISTRY_AUTH }}'
           NIXPKGS_CHANNEL: '${{ matrix.channel }}'