Fix warnings in template generation scripts on Windows

[lbussell]

When running the current scripts on Windows, the script tries to copy files from a container that doesn't exist because Invoke-ImageBuilder.ps1 does not create a container on Windows. As a result it throws some warnings/errors and the command "fails" even though it does what you want:

PS> pwsh .\eng\readme-templates\Get-GeneratedReadmes.ps1
...
Executing: 'docker cp ImageBuilder-20250228105133:/repo/README.aspnet.md C:\...\dotnet-framework-docker/'
Error response from daemon: No such container: ImageBuilder-20250228105133
PS> $?
False

This PR skips the copy on Windows. I made sure it works on all three of Windows PowerShell, PowerShell Core on Windows, and PowerShell Core on Linux.