ci: check for unstaged files

When you do the following
```
$ asdf install
$ npm --version
6.14.16
```

and run:

```
npm install
```

then you get a changed `package-lock.json`.

It is good practice to always check in lock files into version control.
On CI we run `npm install` (among other things). If we make sure that we
do not leave any updates files, this ensures we always commit our lock
files.

.github/workflows/ci.yml

@@ -31,10 +31,19 @@ jobs:
       - name: Update file watchers because of rails settings cached
         run: echo fs.inotify.max_user_watches=524288 | sudo tee -a /etc/sysctl.conf && sudo sysctl -p
 
+      - name: Read .tool-versions
+        uses: marocchino/tool-versions-action@v1
+        id: versions
+
+      - name: Use Node.js ${{ steps.versions.outputs.nodejs}}
+        uses: actions/setup-node@v3
+        with:
+          node-version: ${{ steps.versions.outputs.nodejs}}
+
       - name: Install ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: 3.0.0
+          ruby-version:  ${{ steps.versions.outputs.ruby}}
           bundler-cache: true
 
       - name: Cache NPM dependencies
@@ -64,3 +73,9 @@ jobs:
 
       - name: Run tests
         run: bin/rake spec
+
+      - name: Check unstaged changes
+        run: |
+          git --no-pager diff --color
+          git update-index --refresh
+          git diff-index --quiet HEAD