ci: add -Z stubbing to run kani

cc model-checking/kani#4295
cc model-checking/verify-rust-std@c090506#diff-8348895dea8741e872b7dd41b86e35d72e0f497c56ff6ed5c42bf5958ba2fe45

error: Using the stub_verified attribute requires activating the unstable `stubbing` feature
    --> /home/runner/work/distributed-verification/distributed-verification/verify-rust-std/library/core/src/intrinsics/mod.rs:3583:13
     |
3583 |             #[kani::stub_verified(transmute_unchecked_wrapper)]
     |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
3605 |     should_succeed_no_validity_reqs!(should_succeed_char_to_i32, char, i32);

     |     ----------------------------------------------------------------------- in this macro invocation
     |
     = note: this error originates in the attribute macro `kani::stub_verified` which comes from the expansion of the macro `should_succeed_no_validity_reqs` (in Nightly builds, run with -Z macro-backtrace for more info)

Author: zjp-CN

.github/workflows/test.yml

@@ -54,7 +54,7 @@ jobs:
         autoharness: ["", "-autoharness"]
     if: true
     env:
-      UNSTABLE_ARGS: -Z function-contracts -Z mem-predicates -Z float-lib -Z c-ffi -Z loop-contracts
+      UNSTABLE_ARGS: -Z function-contracts -Z mem-predicates -Z float-lib -Z c-ffi -Z loop-contracts -Z stubbing
       AUTO: ${{ matrix.autoharness }}
     steps:
       - name: Remove unnecessary software to free up disk space