Add `--dry-run` support to `uv self update` (#9829)
See commentary at
https://github.com/astral-sh/uv/issues/9828#issuecomment-2537542100
regarding the limitations and future upstream changes needed.
```
❯ cargo build --features self-update
Compiling uv v0.5.8 (/Users/zb/workspace/uv/crates/uv)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 7.28s
❯ cp ./target/debug/uv ~/.cargo/bin
❯ uv self update --dry-run
info: Checking for updates...
Nothing to do. You're on the latest version of uv (v0.5.8)
❯ uv self update --dry-run 0.5.7
info: Checking for updates...
Would update uv from v0.5.8 to v0.5.7
❯ vi ~/.config/uv/uv-receipt.json # Edit the receipt to think its on an older version
❯ uv self update --dry-run
info: Checking for updates...
Would update uv from v0.5.8 to the latest version
```
---------
Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
