ci: enable flake-check

.github/workflows/nix.yml

@@ -1,5 +1,5 @@
 ---
-
+# yamllint disable-line rule:truthy
 on:
   pull_request:
   push:
@@ -19,10 +19,5 @@ jobs:
         with:
           ignore-missing-flake-lock: false
           fail-mode: true
-      # disabled, because of:
-      # https://github.com/MatthewCroughan/nixinate/issues/12
-      # https://github.com/NixOS/nix/issues/6381
-      # - name: Check Nix flake
-      #   run: nix flake check
-      # - name: Build Nix packages
-      #   run: nix build
+      - name: Check Nix flake
+        run: nix flake check --all-systems