fix(nextjs): Use basePath option for tunnelRoute (#8454)

Author: Luca Forstner

packages/nextjs/src/config/webpack.ts

@@ -865,7 +865,10 @@ function addValueInjectionLoader(
 
   const isomorphicValues = {
     // `rewritesTunnel` set by the user in Next.js config
-    __sentryRewritesTunnelPath__: userSentryOptions.tunnelRoute,
+    __sentryRewritesTunnelPath__:
+      userSentryOptions.tunnelRoute !== undefined
+        ? `${userNextConfig.basePath ?? ''}${userSentryOptions.tunnelRoute}`
+        : undefined,
 
     // The webpack plugin's release injection breaks the `app` directory so we inject the release manually here instead.
     // Having a release defined in dev-mode spams releases in Sentry so we only set one in non-dev mode