treewide: run nginx -t on all nginx config files

overlays/nginx-test.nix

@@ -0,0 +1,11 @@
+final: prev: {
+  writers = prev.writers // {
+    writeNginxConfig = name: text: final.runCommandLocal name {
+      nginxConfig = prev.writers.writeNginxConfig name text;
+      nativeBuildInputs = [ final.nginx ];
+    } ''
+      ln -s "$nginxConfig" "$out"
+      nginx -t -c "$out"
+    '';
+  };
+}