feat(workflows): Switch to a nix-based definition of workflows

workflows/check-workflows.nix

@@ -0,0 +1,20 @@
+{
+  name = "Check workflows";
+  on = {
+    pull_request.branches = [ "main" ];
+    push.paths = [ "workflows/*" ];
+  };
+
+  jobs = {
+    check_workflows = {
+      runs-on = "nix";
+      steps = [
+        { uses = "actions/checkout@v3"; }
+        {
+          name = "Check that the workflows are up to date";
+          run = "nix-shell --run '[ $(git status --porcelain) -eq 0 ]'";
+        }
+      ];
+    };
+  };
+}