Have CI ensure Nix flake lock is up-to-date

Author: mitchmindtree

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
       - uses: cachix/install-nix-action@v16
         with:
           nix_path: nixpkgs=channel:nixos-unstable
-      - run: nix flake check
+      - run: nix flake check --no-update-lock-file
 
   # Checks the nix formatting.
   template-fmt-check: