feat(cli): add --token option to self update command (#7279)

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## Summary

It often reaches the GitHub API rate limit and shows error like `error:
HTTP status client error (403 Forbidden) for url
(https://api.github.com/repos/astral-sh/uv/releases)` when running `uv
self update`.

To bypass this rate limit issue, allow user to pass a GitHub token via
`--token` or `UV_GITHUB_TOKEN` env.

## Test Plan

<!-- How was it tested? -->

---------

Signed-off-by: Frost Ming <me@frostming.com>

crates/uv-cli/src/lib.rs

@@ -427,6 +427,11 @@ pub enum SelfCommand {
 pub struct SelfUpdateArgs {
     /// Update to the specified version. If not provided, uv will update to the latest version.
     pub target_version: Option<String>,
+
+    /// A GitHub token for authentication.
+    /// A token is not required but can be used to reduce the chance of encountering rate limits.
+    #[arg(long, env = "UV_GITHUB_TOKEN")]
+    pub token: Option<String>,
 }
 
 #[derive(Args)]