install-dotnet.ps1 fails behind authenticated proxy: credentials not applied from ProxyAddress

The install-dotnet.ps1 script fails behind authenticated proxy because it does not apply proxy credentials, even when they are provided via the ProxyAddress parameter.

The script constructs its own HttpClientHandler and WebProxy object. It does not extract credentials from the proxy URL or use those set in environment variables. As a result, it sends unauthenticated requests to the proxy, resulting in 407 Proxy Authentication Required errors. Failure logs show that the ProxyAddress with credentials is passed correctly, but the script still receives a 407. Web proxy logs confirm the request reaches the proxy without credentials, indicating that authentication details are not being applied.
Please find error screenshot for reference.

**Environment Context**
The runners are in a restricted network without direct internet access, so all outbound traffic must go through an authenticated proxy.

Inizia in install-dotnet.ps1, nel codice che costruisce HttpClientHandler e WebProxy da ProxyAddress. Traccia come vengono gestite le credenziali del proxy e le variabili d'ambiente, quindi riproduci la richiesta tramite un proxy autenticato. Il lavoro è completato quando le richieste vengono autenticate correttamente e non falliscono più con 407 Proxy Authentication Required.

Scritto dal modello di indicizzazione a partire dal testo della issue.