Support updating `workloadVersion` in `dotnetup`

[nagilson]

Discussed in #54680

^{Originally posted by jjonescz June 10, 2026}
dotnetup install preview --update-global-json updated my sdk.version but not sdk.workloadVersion, now I have to update that manually.
FWIW, I don't need it to run dotnet workload restore or anything, I just want it to find the version number and put it to my global.json.