Automatic toolchain upgrade to nightly-2025-09-28 (#4383)

Update Rust toolchain from nightly-2025-09-27 to nightly-2025-09-28
without any other source changes.

---------

Co-authored-by: celinval <[email protected]>
Co-authored-by: Michael Tautschnig <[email protected]>

Author: 3 people

rust-toolchain.toml

@@ -2,5 +2,5 @@
 # SPDX-License-Identifier: Apache-2.0 OR MIT
 
 [toolchain]
-channel = "nightly-2025-09-27"
+channel = "nightly-2025-09-28"
 components = ["llvm-tools", "rustc-dev", "rust-src", "rustfmt"]

tools/kani-cov/src/args.rs

@@ -10,7 +10,6 @@
 use std::path::PathBuf;
 
 use anyhow::{Result, bail};
-use clap::{arg, command};
 
 use crate::{merge, report, summary};