build: Fix clippy issue

[sboeuf]

Resolves following clippy warning:

warning: variable does not need to be mutable                                                
   --> src/trap.rs:209:9                                                                     
    |                                                                                        
209 |     let mut tf = unsafe { tf_ptr.as_mut().unwrap() };                                  
    |         ----^^                                                                         
    |         |                                                                              
    |         help: remove this `mut`                                                        
    |                                                                                        
    = note: `#[warn(unused_mut)]` on by default                                              

warning: 1 warning emitted

[abrestic-rivos]

hm, why didn't CI catch this?

[sboeuf]

hm, why didn't CI catch this?

Yeah that's a shame it didn't. That's because we need to pass some extra flags to bazel clippy if we want warnings to be treated as errors. Fingers crossed this is something bazel supports.

[glg-rv]

We seem to have lost check for warnings in the move to bazel?

Sorry, passed my usual pre-commit tests because no errors were raised.

[sboeuf]

We seem to have lost check for warnings in the move to bazel?

Sorry, passed my usual pre-commit tests because no errors were raised.

No worries at all, I really hope we can find out how to re-enable this treat warnings as errors in bazel.