Fix MSB4019 error in ConPTY nupkg (#20029)

This commit fixes the following error that occurs when attempting to use the
`Microsoft.Windows.Console.ConPTY` nupkg in a Visual C++ project:

    MSB4019: The imported project "C:\Microsoft.Windows.Console.ConPTY.props" was not found.

Based on the list of reserved and well-known properties, the property
`MSBuildThisProjectDirectory` does not appear to exist.  This PR
replaces it with the correct property, `MSBuildThisFileDirectory`.