(PTFE-3070) Fix redirection web ui

[matthiasL-scality]

Improvements to URL routing:

• Added an if statement to check if $builds_backend is /redirect and, if so, rewrites build URLs to /redirect/$1. This ensures proper handling of builds that require redirection.

Why ?

• Clicking a file in the web UI redirected to the root build listing instead of serving the file. This affected all browser clients (Mozilla User-Agent) navigating via /builds/.
• The rewrite rule introduced in PTFE-3085 used an nginx variable in the replacement string ($builds_backend$1). When $builds_backend resolved to /download/, nginx failed to match the file-download location and fell through to the root listing location instead