Ensure destination deleted if exists before overwriting

[KirillOsenkov]

MSBuild Copy task has recently added this important logic:
https://github.com/dotnet/msbuild/blob/c36a54ed3308d1516ffe1a86b9086c42e4ca996f/src/Tasks/Copy.cs#L288-L294

If the Copy task sees that the destination exists, it deletes it before overwriting. This is important, because if the destination is hardlinked to some other file, previously the Copy task used to effectively overwrite all other hardlinked copies of the content, and this caused corruption that's hard to investigate.

Let's ensure the Copy logic in this repo does the same thing (delete the file before overwriting to "unlink" it in case it's a hardlink)

[erikmav]

I'm explicitly choosing to let the caller decide what delete policy to apply. I fixed this in the Microsoft.Build.CopyOnWrite SDK in the linked PR above.

[erikmav]

New Microsoft.Build.CopyOnWrite package published.