prompt: remove mut from fd variable to remove warning

[rzmk]

warning: variable does not need to be mutable
   --> src\cmd\prompt.rs:162:13
    |
162 |         let mut fd = FileDialog::new()
    |             ----^^
    |             |
    |             help: remove this `mut`
    |
    = note: `#[warn(unused_mut)]` on by default

[jqnatividad]

Thanks for this @rzmk,

But this is because on macOS, rfd has the set_can_create_directories option not available on Windows nor Linux.

I'll tweak it instead so the conditional compilation does not trigger the unused_mut lint.

[jqnatividad]

Fine-tuned the conditional compilation instead @rzmk . Thanks for catching this!

b15c6fd