Setting whitelists using env WHITELIST does not work on Vercel (env not in $_SERVER)

[myl7]

Describe the bug
Setting env WHITELIST does not work on Vercel.
The instance still serves every username.

To Reproduce

1. Fork the repo, go to Vercel, and deploy with a GitHub token
2. Setting env WHITELIST in the Vercel's env panel
3. Open https://<domain>/?user=<some-other-user>
4. The figure still renders. it should be rejected.

Expected behavior
Only usernames in WHITELIST are served.

Screenshots
N/A

Desktop (please complete the following information):

Unrelated to the case.

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

Unrelated to the case.

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context

github-readme-streak-stats/src/whitelist.php

Line 11 in a15ea36

$whitelist = array_map("trim", array_filter(explode(",", $_SERVER["WHITELIST"] ?? "")));

Whitelists are loaded via $_SERVER["WHITELIST"], which does not contain the env particularly on Vercel.
I would suggest changing to $_ENV["WHITELIST"].