`dotnetup` should convert installs based on the `path` winning dotnet, not system `dotnet`

### Prerequisites

- [x] I have read the [dotnetup documentation](https://github.com/dotnet/sdk/tree/release/dnup/documentation/general/dotnetup).
- [x] I have searched for [existing dotnetup issues](https://github.com/dotnet/sdk/issues?q=is%3Aissue%20state%3Aopen%20label%3AArea-dotnetup).
- [x] I have searched for [existing dotnetup discussions](https://github.com/dotnet/sdk/discussions/categories/dotnetup).

### Issue type

Bug report

### Description

https://github.com/dotnet/sdk/pull/55349/changes/BASE..4d8f61a6c4f273def19f919b541996170dac9d13#r3624737095 the default migration prompt that moves over existing installs only uses the 'system path' but really this feature is more helpful for converting from whatever the user has installed and is currently using on the PATH, to `dotnetup`'s hive. I think we should apply the small code change as suggested here - we might need to update some of the wording around the 'init' workflow or 'admin' migration prompt since it's now about whichever dotnet was winning on the path rather than what was in an expected 'admin' install protected location.

### dotnetup version

_No response_