ci: add --no-cache to dotnet restore to prevent stale NuGet resolution

The NuGet global packages cache on CI runners can mask NU1603
warnings by satisfying transitive dependencies from cached packages
rather than performing fresh resolution against the feed.

Adding --no-cache ensures CI always validates package resolution
from scratch, matching the behavior of clean publish agents.

Author: timotheeguerin

.github/workflows/core-ci.yml

@@ -40,7 +40,7 @@ jobs:
         run: pnpm install
 
       - name: Restore .NET dependencies
-        run: dotnet restore
+        run: dotnet restore --no-cache
         working-directory: packages/typespec-vs
 
       - name: Build

eng/tsp-core/pipelines/templates/install.yml

@@ -35,6 +35,6 @@ steps:
     retryCountOnTaskFailure: 3
 
   - ${{ if parameters.useDotNet }}:
-      - script: dotnet restore
+      - script: dotnet restore --no-cache
         displayName: Restore .NET Dependencies
         workingDirectory: packages/typespec-vs