dotnetup: Support custom dotnet roots

We have an `--install-path` parameter, but I'm not sure it works consistently and if you want to use a different install root, you have to specify it on every command.

I think we should probably support specifying the path for the dotnet root / hive that dotnetup uses by default. That should be a setting that is saved in the config, and synced with the `env` commands.

Then I think the scenario for specifying it on an install command line is really just for untracked installs.

Suggested commands:

- `dotnetup env set --install-path ` - Set the install path dotnetup will use for dotnet
- `dotnet sdk install --untracked ` - Install to the specified path in untracked mode. We'd remove the existing `--install-path` parameter on the install commands

Start by tracing the dotnetup env commands and SDK install command handling, then inspect how configuration is persisted and how existing --install-path values flow into those paths. Done means a saved default install root is used and synchronized by env commands, while an explicit path is limited to untracked installs as described.

Written by the indexing model from the issue text.