Kani CI in verify-rust-std suddenly failed with no commit change of kani or verify-rust-std.

This is a bug in kani.

Commits for record:

• model-checking/verify-rust-std@f66ba41
• https://github.com/model-checking/kani/tree/9deeec1bec16c4b1f1173d5d652f99d330a4650f
• cc libcore is out of sync verify-rust-std#465

We can bump toolchain to see if libcore can compile, or fix kani at the rustc version.

NOTE: the current toolchain in kani is nightly-2025-08-16.