Change HOSTNAME variable name in server.js for standalone output

[lucasaba]

Goals

1. Use a distinct and distinguishable environment variable name for server.js

Non-Goals

No response

Background

When using output: "standalone", the built server.js uses the HOSTNAME environment variable to attach the server to a specific IP address or hostname.

If the variable HOSTNAME, which is quite a generic term, is used for other scopes inside the application, there could arise some troubles.

In my specific case, I had to patch the server.js to be able to use a different environment value.

Proposal

The proposal is to use a more specific environment variable name, like NEXT_STANDALONE_HOSTNAME.

[lucasaba]

This PR #85615 should fix the request without introducing breaking changes (suggested by @federico-moretti)