Ensure appPathsManifest variable is inside if block (vercel#60210)

## What?

I noticed that this variable is only used to write the manifest to disk so it doesn't have to be kept around in the function scope.

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### What?

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Closes NEXT-
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Closes NEXT-1980

packages/next/src/build/index.ts

@@ -1497,17 +1497,17 @@ export default async function build(
         ? await readManifest<AppBuildManifest>(appBuildManifestPath)
         : undefined
 
-      let appPathsManifest: Record<string, string> = {}
       const appPathRoutes: Record<string, string> = {}
 
       if (appDir) {
-        appPathsManifest = await readManifest(
+        const appPathsManifest = await readManifest<Record<string, string>>(
           path.join(distDir, SERVER_DIRECTORY, APP_PATHS_MANIFEST)
         )
 
-        Object.keys(appPathsManifest).forEach((entry) => {
-          appPathRoutes[entry] = normalizeAppPath(entry)
-        })
+        for (const key in appPathsManifest) {
+          appPathRoutes[key] = normalizeAppPath(key)
+        }
+
         await writeManifest(
           path.join(distDir, APP_PATH_ROUTES_MANIFEST),
           appPathRoutes