[CI] Fix NuGet restore: use Liara mirror (Nexus TLS chain incomplete)

mirror.soroushasadi.com serves a leaf-only TLS chain (no intermediate).
.NET on Linux does not auto-fetch the intermediate via AIA like Windows
does, so CI/Docker restores fail with NU1301 PartialChain. Switch the
Linux build configs (CI inline config + nuget.docker.config) to the
Liara mirror, which serves a complete chain. Also disable NuGetAudit to
avoid the api.nuget.org (filtered) 100s timeout + NU1900 noise.

Local dev nuget.config keeps Nexus primary (Windows resolves the chain).
Re-add Nexus to the Linux configs once nginx serves fullchain.pem.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nuget.docker.config

@@ -1,9 +1,17 @@
 <?xml version="1.0" encoding="utf-8"?>
+<!--
+  NuGet source for the Docker image build (Linux .NET — runs `dotnet restore`
+  inside the SDK container). Uses the Liara mirror because it serves a complete
+  TLS chain. mirror.soroushasadi.com currently serves a leaf-only chain, which
+  .NET on Linux rejects with PartialChain (Windows auto-fetches the missing
+  intermediate via AIA; Linux does not). Re-add Nexus once nginx serves
+  fullchain.pem for mirror.soroushasadi.com.
+-->
 <configuration>
   <packageSources>
     <clear />
-    <add key="nexus"
-         value="https://mirror.soroushasadi.com/repository/nuget-group/index.json"
+    <add key="liara"
+         value="https://package-mirror.liara.ir/repository/nuget/index.json"
          protocolVersion="3" />
   </packageSources>
   <config>