Give `dotnetup` global json explicit option


### Discussed in https://github.com/dotnet/sdk/discussions/54648

<div type='discussions-op-text'>

<sup>Originally posted by **jjonescz** June  9, 2026</sup>
Can we make `dotnetup sdk install` have some explicit option to say "install from global.json"? I don't like that it's only implied now.</div>