fix: new clippy warnings via stable rustc

[RobertMueller2]

error: redundant reference in `format!` argument
   --> src/sketch_board.rs:533:53
    |
533 |                     &format!("File saved to '{}'.", &output_filename),
    |                                                     ^^^^^^^^^^^^^^^^ help: remove the redundant `&`: `output_filename`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.97.0/index.html#useless_borrows_in_formatting
    = note: `-D clippy::useless-borrows-in-formatting` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::useless_borrows_in_formatting)]`

error: redundant reference in `format!` argument
   --> src/sketch_board.rs:610:65
    |
610 | ...                   &format!("File saved to '{}'.", &output_filename),
    |                                                       ^^^^^^^^^^^^^^^^ help: remove the redundant `&`: `output_filename`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.97.0/index.html#useless_borrows_in_formatting

error: redundant reference in `eprintln!` argument
   --> src/main.rs:437:13
    |
437 |             &path.display()
    |             ^^^^^^^^^^^^^^^ help: remove the redundant `&`: `path.display()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.97.0/index.html#useless_borrows_in_formatting

error: redundant reference in `eprintln!` argument
   --> src/main.rs:447:17
    |
447 |                 &path.display(),
    |                 ^^^^^^^^^^^^^^^ help: remove the redundant `&`: `path.display()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.97.0/index.html#useless_borrows_in_formatting