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

[Copilot]

Description

This PR updates the .github/workflows/copilot-setup-steps.yml workflow to use the global-json-file input for the actions/setup-dotnet@v5 action, instead of hardcoding the .NET SDK version directly in the workflow file.

Changes

• Changed from explicitly specifying dotnet-version: 10.0.100-rc.1.25451.107 and dotnet-quality: preview to using global-json-file: global.json
• Updated the step name from "Install .NET 10.x" to "Install .NET SDK" to make it version-agnostic

Benefits

1. Single source of truth: The .NET SDK version is now centrally managed in global.json, which is already the authoritative source for the SDK version across the repository
2. Easier maintenance: When the SDK version needs to be updated, only global.json needs to be changed, and the workflow will automatically pick up the new version
3. Follows best practices: Aligns with the GitHub Actions setup-dotnet documentation
4. Automatic configuration: The action will automatically respect all settings from global.json, including allowPrerelease, rollForward, and other SDK configuration options

Example

Before:

- name: Install .NET 10.x
  uses: actions/setup-dotnet@v5
  with:
    dotnet-version: |
      10.0.100-rc.1.25451.107
    dotnet-quality: preview

After:

- name: Install .NET SDK
  uses: actions/setup-dotnet@v5
  with:
    global-json-file: global.json

This ensures the workflow always uses the SDK version specified in the repository's global.json file, making version management more maintainable and consistent.

Original prompt

Please update the copilot-setup-steps.yml to use the version of the .NET SDK as specified in the repos' global.json, as described in the docs at https://github.com/actions/setup-dotnet?tab=readme-ov-file#using-the-global-json-file-input

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[stephentoub]

@copilot, please run all tests in the repo.