Add brew specific message for `uv self update`

[terror]

Resolves https://github.com/astral-sh/uv/issues/16833

uv self update could error with a better message if it knows the user has installed it with brew. This diff adds an InstallSource we can use the detect where a uv binary comes from and augment error messages with better context. We're only adding brew for now, but it could easily be extend with other detection heuristics.