Update copilot-setup-steps.yml to use global.json for .NET SDK version (modelcontextprotocol#888)

* Initial plan

* Update copilot-setup-steps.yml to use global.json for .NET SDK version

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

.github/workflows/copilot-setup-steps.yml

@@ -13,12 +13,10 @@ jobs:
     steps:
       - uses: actions/checkout@v5
 
-      - name: Install .NET 10.x
+      - name: Install .NET SDK
         uses: actions/setup-dotnet@v5
         with:
-          dotnet-version: |
-            10.0.100-rc.1.25451.107
-          dotnet-quality: preview
+          global-json-file: global.json
 
       - name: dotnet --info
         run: dotnet --info