Include .NET 10 global install in copilot setup steps

[danmoseley]

Description

Install .NET 10 globally in the copilot setup steps. It's not required for build, but it is required for (at least) the NuGet MCP server, which I plan to enable. This change adds less than 10 seconds to the 1 minute setup.

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Link to aspire-docs issue (consider using one of the following templates):
      • New (or update) doc-idea template
      • New breaking-change template
      • New diagnostic template
  • No

[Copilot]

Pull Request Overview

This PR adds .NET 10 installation to the GitHub Actions workflow for Copilot setup to support MCP servers (specifically the NuGet MCP server). The change ensures that .NET 10 is globally available during Copilot operations while maintaining the existing build requirements.

Key Changes:

• Adds .NET 10.x preview installation step to the Copilot setup workflow
• Includes a diagnostic step to log .NET installation information

[radical]

The restore step should be installing it. Maybe we need to add ${{ github.workspace }}/.dotnet to $GITHUB_PATH?

[danmoseley]

as discussed offline, 'dotnet.sh' puts down P5 for its local install at the moment, which is too old, but more importantly doesn't seem to put it on the path. we can investigate later.