ci: restore ci.nix

The cachix action doesn't support flakes yet
See cachix/cachix-action#47

.github/workflows/cachix.yml

@@ -31,4 +31,4 @@ jobs:
         authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
         # FIXME: This is necessary since we used 'nix profile' before
         installCommand: "nix profile install --accept-flake-config nixpkgs#cachix"
-    - run: nix-fast-build --skip-cached --no-nom --flake ".#nixosConfigurations.matebook.config.system.build.toplevel"
+    - run: nix-fast-build --skip-cached --no-nom ./ci.nix

.github/workflows/update.yml

@@ -31,7 +31,7 @@ jobs:
         id: build
         run: |
           nix flake update
-          nix-fast-build --skip-cached --no-nom --flake ".#nixosConfigurations.matebook.config.system.build.toplevel"
+          nix-fast-build --skip-cached --no-nom ./ci.nix
       - name: Commit flake.lock on success
         if: steps.build.outcome == 'success'
         uses: stefanzweifel/git-auto-commit-action@v5

ci.nix

@@ -0,0 +1,6 @@
+let
+  outputs = builtins.getFlake (toString ./.);
+  pkgs = outputs.inputs.nixpkgs;
+  drvs = pkgs.lib.collect pkgs.lib.isDerivation outputs.nixosConfigurations.matebook.config.system.build.toplevel;
+in
+  drvs