Update Copilot setup to use .NET SDK version from tools.dotnet

[Copilot]

Our repository uses a non-standard approach to specifying the .NET SDK version in global.json. Instead of using the standard sdk.version property, we use the Arcade-specific tools.dotnet element.

This PR updates the Copilot setup steps to:

1. Extract the SDK version from tools.dotnet using jq
2. Pass the extracted version to setup-dotnet using the dotnet-version parameter

Before this change, Copilot was using the global-json-file parameter which doesn't understand our custom structure, potentially using an incorrect SDK version.

      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+      
+      - name: Extract SDK version from global.json
+        id: sdk-version
+        run: |
+          SDK_VERSION=$(jq -r '.tools.dotnet' global.json)
+          echo "SDK_VERSION=$SDK_VERSION" >> $GITHUB_ENV
+          echo "Extracted .NET SDK version: $SDK_VERSION"

      - uses: actions/setup-dotnet@67a3573c9a986a3f9c594539f4ab511d57bb3ce9 # 4.3.1
        with:
-          global-json-file: global.json
+          dotnet-version: ${{ env.SDK_VERSION }}

Fixes #11890.

---

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[dotnet-policy-service]

Hello @@copilot, I noticed that you’re changing an .swr file or any file under src/Package/MSBuild.VSSetup.. Please make sure to validate this change by an experimental VS insertion. This is accomplished by pushing to an exp/* branch, which requires write permissions to this repo.

[Copilot]

Pull Request Overview

This PR updates the GitHub Actions workflow to pull the .NET SDK version from the custom tools.dotnet property in global.json and supply it directly to actions/setup-dotnet.

• Added a step to extract the SDK version using jq and set it in the environment
• Switched setup-dotnet from global-json-file to the extracted dotnet-version parameter

Comments suppressed due to low confidence (2)

.github/workflows/copilot-setup-steps.yml:20

• [nitpick] Ensure this new step’s indentation matches the existing steps (two extra spaces), so it aligns under steps: with other - uses: entries.

- name: Extract SDK version from global.json

.github/workflows/copilot-setup-steps.yml:23

• [nitpick] Verify that jq is available on the runner (or add an install step) to avoid failures on environments where jq isn’t preinstalled.

SDK_VERSION=$(jq -r '.tools.dotnet' global.json)

[rainersigwald]

Looks like runtime actually does this a different way; we should probably do the same:

https://github.com/dotnet/runtime/blob/4bdcb8d4b40cae07b3a654d378e0b58bf7d62dd7/.github/workflows/copilot-setup-steps.yml#L40-L44

[Copilot]

Looks like runtime actually does this a different way; we should probably do the same:

https://github.com/dotnet/runtime/blob/4bdcb8d4b40cae07b3a654d378e0b58bf7d62dd7/.github/workflows/copilot-setu...

Good point! I've updated the approach to follow runtime's pattern. Instead of using actions/setup-dotnet, we now rely on the build system to install .NET to the local .dotnet directory and add it to PATH, just like runtime does. This is simpler and more consistent with how other repositories handle this. See commit 556a436.