feat: redirect to build.npmx.dev in staging

[patak-cat]

Describe the feature

The production app will now link to the community discord (as the chat.npmx.dev link is pointing to it now):

• feat: chat.npmx.dev + build.npmx.dev #1774

For builders testing main.npmx.dev (staging), we could link instead to build.npmx.dev

Additional information

• Would you be willing to help implement this feature?

Final checks

• Read the contribution guide.
• Check existing issues.