SConstruct : repair symlinks on Windows
SConstruct : repair symlinks on Windows

- Windows only supports symlinks when the user is an admin or they have enabled developer mode. Most users don't have admin rights, and developer mode has additional implications that may not be desirable. We copy the symlink source when building to ensure the files have the correct content.
- Note that builds running on Github workflows are running as admin, so symlinks are correctly made. Testing for the existence of the source file ensures we don't attempt to copy bogus source files.

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