Add brew specific message for `uv self update` (#16838)
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.
