Setup NPM with an empty package.json file

Note that, even if we're excluding the `node_modules/` folder from
version control, we aren't adding it to Capistrano's shared folders
because, when `node_modules` is a symbolic link, NPM removes it when
running `npm install`.

.github/workflows/tests.yml

@@ -42,7 +42,10 @@ jobs:
       - name: Setup NPM
         uses: actions/setup-node@v3
         with:
+          cache: "npm"
           node-version-file: ".node-version"
+      - name: Install node packages
+        run: npm clean-install
       - name: Copy secrets and database files
         run: for i in config/*.example; do cp "$i" "${i/.example}"; done
       - name: Setup database