Exit early on `self update` if global `--offline` is set (#13663)
## Summary
Resolves https://github.com/astral-sh/uv/issues/13580.
`uv self update --offline` should fail and exit early because
self-updating requires network connection.
## Test Plan
A snapshot test is added.
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Co-authored-by: Aria Desires <aria.desires@gmail.com>
Co-authored-by: konsti <konstin@mailbox.org>
