Fix copying "dev-404-page.js" to the cache folder on Windows

Fixes #2819

Author: kripod

packages/gatsby/src/bootstrap/load-plugins.js

@@ -133,6 +133,9 @@ module.exports = async (config = {}) => {
   }
 
   // Add internal plugins
+  plugins.push(
+    processPlugin(path.join(__dirname, `../internal-plugins/dev-404-page`))
+  )
   plugins.push(
     processPlugin(
       path.join(__dirname, `../internal-plugins/component-page-creator`)
@@ -148,9 +151,6 @@ module.exports = async (config = {}) => {
       path.join(__dirname, `../internal-plugins/internal-data-bridge`)
     )
   )
-  plugins.push(
-    processPlugin(path.join(__dirname, `../internal-plugins/dev-404-page`))
-  )
   plugins.push(
     processPlugin(path.join(__dirname, `../internal-plugins/prod-404`))
   )

packages/gatsby/src/internal-plugins/dev-404-page/gatsby-node.js

@@ -8,7 +8,7 @@ exports.createPages = async ({ store, boundActionCreators }) => {
     const currentPath = path.join(__dirname, `./raw_dev-404-page.js`)
     const newPath = path.join(program.directory, `.cache`, `dev-404-page.js`)
 
-    fs.copySync(currentPath, newPath)
+    await fs.copy(currentPath, newPath)
 
     createPage({
       component: newPath,