cranelift: fix build warning

[abrown]

In #4375 we introduced a code pattern that appears as a warning when
building the cranelift-interpreter crate:

warning: cannot borrow `*state` as mutable because it is also borrowed as immutable
   --> cranelift/interpreter/src/step.rs:412:13
    |
47  |     let arg = |index: usize| -> Result<V, StepError> {
    |               -------------------------------------- immutable borrow occurs here
48  |         let value_ref = inst_context.args()[index];
49  |         state
    |         ----- first borrow occurs due to use of `*state` in closure
...
412 |             state.set_pinned_reg(arg(0)?);
    |             ^^^^^^^^^^^^^^^^^^^^^---^^^^^
    |             |                    |
    |             |                    immutable borrow later used here
    |             mutable borrow occurs here
    |
    = note: `#[warn(mutable_borrow_reservation_conflict)]` on by default
    = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
    = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>

This change fixes the warning.

[abrown]

cc: @afonso360

[jameysharp]

That makes sense!