[ppr] Data route fix (vercel#66190)

Fixes a typo in the logic for checking if it should add the prefetch
data doute.

packages/next/src/build/index.ts

@@ -2793,7 +2793,7 @@ export default async function build(
                 let prefetchDataRoute: string | null | undefined
                 // We write the `.prefetch.rsc` when the app has PPR enabled, so
                 // always add the prefetch data route to the manifest.
-                if (!isRouteHandler && experimentalPPR) {
+                if (!isRouteHandler && isAppPPREnabled) {
                   prefetchDataRoute = path.posix.join(
                     `${normalizedRoute}${RSC_PREFETCH_SUFFIX}`
                   )