Copilot setup (#11885)

Follow the example of https://github.com/dotnet/aspnetcore/blob/aebd07af9854541970d0c05d99dc2e6e8bd2fdd4/.github/workflows/copilot-setup-steps.yml
to restore NuGet packages before Copilot starts doing anything.

Author: rainersigwald

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

@@ -0,0 +1,25 @@
+name: "Copilot Setup Steps"
+
+# Allow testing of the setup steps from your repository's "Actions" tab.
+on: workflow_dispatch
+
+jobs:
+  # The job MUST be called `copilot-setup-steps` or it will not be picked up by Copilot.
+  # See https://docs.github.com/en/copilot/customizing-copilot/customizing-the-development-environment-for-copilot-coding-agent
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+
+    # You can define any steps you want, and they will run before the agent starts.
+    # If you do not check out your code, Copilot will do this for you.
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+
+      - uses: actions/setup-dotnet@67a3573c9a986a3f9c594539f4ab511d57bb3ce9 # 4.3.1
+        with:
+          global-json-file: global.json
+
+      - name: Restore MSBuild
+        run: ./eng/common/build.sh --restore