Add submodule checkout to copilot-setup-steps.yml

[wtgodbe]

Copilot fails to build certain projects without the MessagePack submodule checked out

[Copilot]

Pull request overview

This PR adds submodule checkout configuration to the copilot-setup-steps.yml GitHub Actions workflow file. The repository uses git submodules (googletest and MessagePack-CSharp) which need to be initialized for certain build operations, and this change ensures they are available when Copilot runs setup steps.

• Adds the submodules: 'true' parameter to the actions/checkout step to enable automatic submodule initialization
• Ensures the workflow has access to code in the src/submodules directory for build operations

[Copilot]

The submodules parameter should be a boolean value 'true' or 'false', or a string specifying the checkout strategy ('recursive'). While the string 'true' will work, it's more idiomatic in GitHub Actions to use the boolean true without quotes, or use 'recursive' to explicitly check out all nested submodules. Looking at other workflows in the repository (e.g., validate-npm-package-lock-json.yml uses submodules: false), the convention is to use boolean values without quotes.

Suggested change

	submodules: 'true'
	submodules: true