Fix build failure caused by mapbox-gl

Instructions here:
gatsbyjs/gatsby#6667 (comment)

gatsby-node.js

@@ -3,6 +3,21 @@ const path = require('path')
 const { createFilePath } = require('gatsby-source-filesystem')
 const { fmImagesToRelative } = require('gatsby-remark-relative-images')
 
+exports.onCreateWebpackConfig = ({ stage, loaders, actions }) => {
+  if (stage === 'build-html') {
+    actions.setWebpackConfig({
+      module: {
+        rules: [
+          {
+            test: /mapbox-gl/,
+            use: loaders.null(),
+          },
+        ],
+      },
+    })
+  }
+}
+
 exports.createPages = ({ actions, graphql }) => {
   const { createPage } = actions