Publish: Hint at --skip-existing -> --check-url transition (#8803)

See https://github.com/astral-sh/uv/pull/8531#issuecomment-2442698889,
we hint users coming from twine to use `--check-url` instead.

> `uv publish` does not support `--skip-existing`, use `--check-url`
with the simple index URL instead.

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

crates/uv-cli/src/lib.rs

@@ -4882,8 +4882,11 @@ pub struct PublishArgs {
     /// file succeeds even without `--check-url`, while most other indexes error.
     ///
     /// The index must provide one of the supported hashes (SHA-256, SHA-384, or SHA-512).
-    #[arg(long,env = EnvVars::UV_PUBLISH_CHECK_URL)]
+    #[arg(long, env = EnvVars::UV_PUBLISH_CHECK_URL)]
     pub check_url: Option<IndexUrl>,
+
+    #[arg(long, hide = true)]
+    pub skip_existing: bool,
 }
 
 /// See [PEP 517](https://peps.python.org/pep-0517/) and