Bump MSRV to 1.84 (#12670)

## Summary

Closes https://github.com/astral-sh/uv/issues/12649.

crates/uv-build-frontend/src/lib.rs

@@ -568,7 +568,7 @@ impl SourceBuild {
                                 nor `setup.cfg` are present in the directory",
                                 source_tree.simplified_display().cyan(),
                             );
-                        };
+                        }
                     }
                     default_backend.clone()
                 };