Fix System.Security.Cryptography.Xml dependency

[Tratcher]

#47894 did some infrastructure changes that broke source build when trying to insert into the installer dotnet/installer#16275.

error NU1605: Warning As Error: Detected package downgrade: System.Security.Cryptography.Xml from 8.0.0-preview.5.23228.8 to 6.0.1. Reference the package directly from the project to select a different version.

We reverted part of the change here (#48029) to unblock the insertion. What's the correct fix?

[wtgodbe]

Nothing more is needed here as far as I know