`cargo init` in the home directory can lead to confusing states

### Problem

```console
$ cd ~
$ cargo init
```

Easy to do when experimenting as a new user but could then cause problems with file discovery (e.g. #6646)

### Proposed Solution

Error out

Note: we've previously decided that this was a UX command and changes in behavior that break programmatic use are not blockers.

### Notes

_No response_