feat: add eval action

.forgejo/workflows/eval.yml

@@ -0,0 +1,20 @@
+name: Evaluate configurations
+
+on:
+  push:
+
+jobs:
+  eval:
+    name: Evaluate configurations
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v14
+
+      - name: Evaluate
+        run: |
+          nix eval .#nixosConfigurations --apply 'builtins.mapAttrs (name: cfg: cfg.config.system.build.toplevel)'