fix(nextjs): Move server-only resolution to runtime

.changeset/lucky-olives-explode.md

@@ -0,0 +1,5 @@
+---
+"@clerk/nextjs": patch
+---
+
+Fixes regression on `@clerk/nextjs/server` (introduced on https://github.com/clerk/javascript/pull/3758) where `server-only` module was being resolved in runtimes without `react-server` available, such as `getAuth` on `getServerSideProps`.

packages/nextjs/src/app-router/server/auth.ts

@@ -1,5 +1,3 @@
-import 'server-only';
-
 import type { AuthObject, RedirectFun } from '@clerk/backend/internal';
 import { constants, createClerkRequest, createRedirect } from '@clerk/backend/internal';
 import { notFound, redirect } from 'next/navigation';
@@ -16,6 +14,8 @@ import { buildRequestLike } from './utils';
 type Auth = AuthObject & { protect: AuthProtect; redirectToSignIn: RedirectFun<ReturnType<typeof redirect>> };
 
 export const auth = (): Auth => {
+  require('server-only');
+
   const request = buildRequestLike();
   const authObject = createGetAuth({
     debugLoggerName: 'auth()',

packages/nextjs/src/app-router/server/currentUser.ts

@@ -1,11 +1,11 @@
-import 'server-only';
-
 import type { User } from '@clerk/backend';
 
 import { clerkClient } from '../../server/clerkClient';
 import { auth } from './auth';
 
 export async function currentUser(): Promise<User | null> {
+  require('server-only');
+
   const { userId } = auth();
   if (!userId) {
     return null;