[msbuild] Copy some ILLink output back to Windows when building remotely.

[rolfbjarne]

This is needed when building iOS apps using CoreCLR or NativeAOT remotely (in
the .NET 11 branch), and I'm adding these changes into main to ease upcoming
changes to these files in main.

No tests here, but the net11.0 branch already contains tests that will verify this.

Ref: #24521

[Copilot]

Pull request overview

This PR updates the Windows-remote build pipeline for iOS so that selected ILLink outputs produced on the remote Mac are copied back to Windows, enabling subsequent Windows-side steps (notably for CoreCLR/NativeAOT scenarios).

Changes:

• Adds a new CopyToWindows switch to the Xamarin.MacDev.Tasks.ILLink task and wires it up from the Windows remote targets when UseMonoRuntime=false.
• Introduces helper plumbing in XamarinTask to support copying files back to Windows using a TaskRunner instance from remote execution.
• Updates ILLink’s remote-execution behavior to download linked outputs (and some linker cache outputs) to Windows after remote completion.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
msbuild/Xamarin.iOS.Tasks.Windows/Xamarin.iOS.Common.After.targets	Enables ILLink output copying back to Windows for non-Mono runtime builds by setting/passing CopyToWindows.
msbuild/Xamarin.MacDev.Tasks/Tasks/XamarinTask.cs	Adds a static helper overload to copy files back to Windows given an explicit MSBuild Task instance.
msbuild/Xamarin.MacDev.Tasks/Tasks/ILLink.cs	Implements CopyToWindows behavior for remote ILLink execution and adjusts output-file placeholder creation logic accordingly.

[vs-mobiletools-engineering-service2]

✅ [PR Build #c202a58] Build passed (Detect API changes) ✅

Pipeline on Agent
Hash: c202a583cf97602c1246127fee50e357ee02b25b [PR build]