Use specific .NET version for NuGet operations

Introduces DOTNET_NUGET_VERSION environment variable to specify the .NET version for NuGet package operations, decoupling it from the SDK version in global.json. Updates setup-dotnet steps to use this version for package-related jobs.

Author: Sergio0694

.github/workflows/ci-build.yml

@@ -21,6 +21,11 @@ env:
   IS_PR: ${{ startsWith(github.ref, 'refs/pull/') }}
   IS_RELEASE: ${{ startsWith(github.ref, 'refs/heads/rel/') }}
 
+  # The version of .NET to use just for NuGet package operations.
+  # This doesn't have to be exactly in sync with the .NET SDK
+  # version defined in 'global.json', as it's not used for builds.
+  DOTNET_NUGET_VERSION: ${{ '9.0.x' }}
+
 jobs:
 
   # Build the solution, run all tests, push packages to the PR feed.
@@ -105,7 +110,7 @@ jobs:
       - name: Install .NET SDK
         uses: actions/setup-dotnet@v5
         with:
-          global-json-file: global.json
+          dotnet-version: ${{ env.DOTNET_NUGET_VERSION }}
 
       - name: Download packages list
         uses: actions/download-artifact@v5
@@ -165,7 +170,7 @@ jobs:
       - name: Install .NET SDK
         uses: actions/setup-dotnet@v5
         with:
-          global-json-file: global.json
+          dotnet-version: ${{ env.DOTNET_NUGET_VERSION }}
 
       - name: Download signed packages for .NCT
         uses: actions/download-artifact@v5