Re-add internal env for providing page paths (#70235)

This re-adds internal env variables to allow providing page paths
directly for debugging purposes. This is NOT a public API and should not
be leveraged outside of internal Next.js debugging.

x-ref: #67134
x-ref: [slack
thread](https://vercel.slack.com/archives/C04DUD7EB1B/p1726519130567199)

packages/next/src/build/index.ts

@@ -912,8 +912,13 @@ export default async function build(
         appDir
       )
 
-      let pagesPaths =
-        !appDirOnly && pagesDir
+      const providedPagePaths: string[] = JSON.parse(
+        process.env.NEXT_PRIVATE_PAGE_PATHS || '[]'
+      )
+
+      let pagesPaths = Boolean(process.env.NEXT_PRIVATE_PAGE_PATHS)
+        ? providedPagePaths
+        : !appDirOnly && pagesDir
           ? await nextBuildSpan.traceChild('collect-pages').traceAsyncFn(() =>
               recursiveReadDir(pagesDir, {
                 pathnameFilter: validFileMatcher.isPageFile,
@@ -1008,18 +1013,24 @@ export default async function build(
           }
 
       if (appDir) {
-        let appPaths = await nextBuildSpan
-          .traceChild('collect-app-paths')
-          .traceAsyncFn(() =>
-            recursiveReadDir(appDir, {
-              pathnameFilter: (absolutePath) =>
-                validFileMatcher.isAppRouterPage(absolutePath) ||
-                // For now we only collect the root /not-found page in the app
-                // directory as the 404 fallback
-                validFileMatcher.isRootNotFound(absolutePath),
-              ignorePartFilter: (part) => part.startsWith('_'),
-            })
-          )
+        const providedAppPaths: string[] = JSON.parse(
+          process.env.NEXT_PRIVATE_APP_PATHS || '[]'
+        )
+
+        let appPaths = Boolean(process.env.NEXT_PRIVATE_APP_PATHS)
+          ? providedAppPaths
+          : await nextBuildSpan
+              .traceChild('collect-app-paths')
+              .traceAsyncFn(() =>
+                recursiveReadDir(appDir, {
+                  pathnameFilter: (absolutePath) =>
+                    validFileMatcher.isAppRouterPage(absolutePath) ||
+                    // For now we only collect the root /not-found page in the app
+                    // directory as the 404 fallback
+                    validFileMatcher.isRootNotFound(absolutePath),
+                  ignorePartFilter: (part) => part.startsWith('_'),
+                })
+              )
 
         if (appPaths && isFullFlyingShuttle) {
           await nextBuildSpan