Add yarn.nix checks

cd to correct directories this time

"diff" exits with exit code 1 iff the files aren't the same

.github/workflows/ci.yml

@@ -43,6 +43,11 @@ jobs:
     - uses: cachix/install-nix-action@v22
       with:
         nix_path: nixpkgs=channel:nixos-unstable
+    - name: Check each yarn.nix up to date
+      run: |
+        cd client && nix-shell -p yarn2nix --run "yarn2nix > new_yarn.nix" && diff new_yarn.nix yarn.nix
+        cd client/goal_view_ui && nix-shell -p yarn2nix --run "yarn2nix > new_yarn.nix" && diff new_yarn.nix yarn.nix
+        cd client/search-ui && nix-shell -p yarn2nix --run "yarn2nix > new_yarn.nix" && diff new_yarn.nix yarn.nix
     - run: nix develop .#vscoq-language-server-${{ matrix.coq }} -c bash -c "cd language-server && dune build"
     - run: nix develop .#vscoq-client -c bash -c "cd client && yarn run install:all && yarn run build:all && yarn run compile"
     - run: xvfb-run nix develop .#vscoq-client -c bash -c "cd client && yarn test"