Fixes 'next export' when exportPathMap function depends on runtime config (#4339)

server/export.js

@@ -81,9 +81,6 @@ export default async function (dir, options, configuration) {
     }
   }
 
-  const exportPathMap = await nextConfig.exportPathMap(defaultPathMap)
-  const exportPaths = Object.keys(exportPathMap)
-
   // Start the rendering process
   const renderOpts = {
     dir,
@@ -116,6 +113,9 @@ export default async function (dir, options, configuration) {
     nextExport: true
   }
 
+  const exportPathMap = await nextConfig.exportPathMap(defaultPathMap)
+  const exportPaths = Object.keys(exportPathMap)
+
   for (const path of exportPaths) {
     log(`> exporting path: ${path}`)
     if (!path.startsWith('/')) {