feat(gatsby): use production React for dev-ssr when CI=true (#28728)

* feat(gatsby): use production React for dev-ssr when CI=true

* Use @pieh's suggestion for switching to the prod versions of React/ReactDOM

* Don't need this now

(cherry picked from commit bd6b899)

packages/gatsby/src/utils/webpack.config.js

@@ -1,5 +1,6 @@
 require(`v8-compile-cache`)
 
+const { isCI } = require(`gatsby-core-utils`)
 const crypto = require(`crypto`)
 const fs = require(`fs-extra`)
 const path = require(`path`)
@@ -675,6 +676,23 @@ module.exports = async (
 
     config.externals = [
       function (context, request, callback) {
+        if (
+          stage === `develop-html` &&
+          isCI() &&
+          process.env.GATSBY_EXPERIMENTAL_DEV_SSR
+        ) {
+          if (request === `react`) {
+            callback(null, `react/cjs/react.production.min.js`)
+            return
+          } else if (request === `react-dom/server`) {
+            callback(
+              null,
+              `react-dom/cjs/react-dom-server.node.production.min.js`
+            )
+            return
+          }
+        }
+
         const external = isExternal(request)
         if (external !== null) {
           callback(null, external)