Fix clippy lint on rust 1.91

[Earlopain]

Compiling ruby-prism v1.6.0 (/home/runner/work/prism/prism/rust/ruby-prism)
error: implicitly cloning a `String` by calling `to_string` on its dereferenced type
   --> ruby-prism/build.rs:148:14
    |
148 |         _ => kind.to_string(),
    |              ^^^^^^^^^^^^^^^^ help: consider using: `kind.clone()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.91.0/index.html#implicit_clone
    = note: `-D clippy::implicit-clone` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::implicit_clone)]`