nix: Enable flake support

modules/nix.nix

@@ -24,6 +24,9 @@ in
   };
 
   nix = {
+    # nix with flake support
+    package = pkgs.nixUnstable;
+
     nixPath = [
       "/var/src" # pinned nixpkgs and configuration
       "nixpkgs=/var/src/nixpkgs" # for nix run
@@ -45,6 +48,8 @@ in
       # Binary cache upload
       secret-key-files = ${config.sops.secrets.binary-cache-secret-key.path}
       netrc-file = ${config.sops.secrets.nix-netrc.path}
+
+      experimental-features = nix-command flakes
     '' + lib.optionalString config.sbruder.full ''
       # Keep output of derivations with gc root
       keep-outputs = true