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>

crates/uv-cli/src/lib.rs

@@ -589,6 +589,10 @@ pub struct SelfUpdateArgs {
     /// A token is not required but can be used to reduce the chance of encountering rate limits.
     #[arg(long, env = EnvVars::UV_GITHUB_TOKEN)]
     pub token: Option<String>,
+
+    /// Run without performing the update.
+    #[arg(long)]
+    pub dry_run: bool,
 }
 
 #[derive(Args)]