Use the Node version specified in nvmrc to build

We're updating from Node 18 to Node 22 LTS. Using the `.nvmrc` file to keep this step consistent and avoid having to change it in multiple places in the future.

.github/workflows/preview-docs.yml

@@ -54,7 +54,7 @@ jobs:
       - name: Install node
         uses: actions/setup-node@v4
         with:
-          node-version: 18
+          node-version-file: modal/.nvmrc
 
       - name: Install node packages
         run: npm ci --include=dev