[static-build] Patch the gatsby-node config file to invoke @vercel/gatsby-plugin-vercel-builder (#9308)

We need to ensure that `@vercel/gatsby-plugin-vercel-builder` is executed as the very last plugin, which Gatsby itself doesn't really support in the plugins listing. So instead we'll patch the `gatsby-node` file to invoke the plugin in the project's own `onPostBuild()` hook, which does get executed last. The patching is similar to how is done with the `gatsby-config` file.

packages/static-build/test/fixtures/gatsby-v4-pnpm/gatsby-config.ts

@@ -1,4 +1,6 @@
-module.exports = {
+import type { GatsbyConfig } from 'gatsby';
+
+const config: GatsbyConfig = {
   siteMetadata: {
     title: `Gatsby Default Starter`,
     description: `Kick off your next, great Gatsby project with this default starter. This barebones starter ships with the main Gatsby configuration files you might need.`,
@@ -33,3 +35,5 @@ module.exports = {
     },
   ],
 };
+
+export default config;

packages/static-build/test/fixtures/gatsby-v4-pnpm/gatsby-node.js

@@ -1,9 +0,0 @@
-exports.createPages = async ({ actions }) => {
-  const { createPage } = actions;
-  createPage({
-    path: '/using-dsg',
-    component: require.resolve('./src/templates/using-dsg.js'),
-    context: {},
-    defer: true,
-  });
-};

packages/static-build/test/fixtures/gatsby-v4-pnpm/gatsby-node.ts

@@ -0,0 +1,12 @@
+import path from 'path';
+import type { GatsbyNode } from 'gatsby';
+
+export const createPages: GatsbyNode['createPages'] = async ({ actions }) => {
+  const { createPage } = actions;
+  createPage({
+    path: '/using-dsg',
+    component: path.resolve('./src/templates/using-dsg.js'),
+    context: {},
+    defer: true,
+  });
+};

packages/static-build/test/fixtures/gatsby-v4/asset.txt

@@ -0,0 +1 @@
+asset that was added in `onPostBuild`

packages/static-build/test/fixtures/gatsby-v4/gatsby-node.js

@@ -1,3 +1,5 @@
+const fs = require('fs');
+
 exports.createPages = async ({ actions }) => {
   const { createPage } = actions;
   createPage({
@@ -8,3 +10,6 @@ exports.createPages = async ({ actions }) => {
   });
 };
 
+exports.onPostBuild = async () => {
+  await fs.promises.copyFile('asset.txt', 'public/asset.txt');
+};

packages/static-build/test/fixtures/gatsby-v4/probes.json

@@ -21,6 +21,10 @@
     {
       "path": "/page-data/using-dsg/page-data.json",
       "mustContain": "\"componentChunkName\":\"component---src-templates-using-dsg-js\",\"path\":\"/using-dsg\""
+    },
+    {
+      "path": "/asset.txt",
+      "mustContain": "asset that was added in `onPostBuild`"
     }
   ]
 }