style: Remove needless borrow

[caspermeijn]

warning: the borrowed expression implements the required traits
   --> protobuf/build.rs:143:14
    |
143 | ...rg(&format!("-DCMAKE_INSTALL_PREFIX={}", prefix_dir.display(...
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: change this to: `format!("-DCMAKE_INSTALL_PREFIX={}", prefix_dir.display())`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrows_for_generic_args
    = note: `#[warn(clippy::needless_borrows_for_generic_args)]` on by default