Guard Version.Details.xml in non internal builds

[ViktorHofer]

• Is this issue blocking: no
• Is this issue causing unreasonable pain: no

During a merge conflict in dotnet/runtime, the Version.Details.xml was incorrectly modified because of a missing tag. More details here: dotnet/runtime#32748.

Can we protect that path by adding the parsing step into the non-public builds?

cc @akoeplinger