style: Use variables directly in the format! string

[caspermeijn]

Prevent clippy warnings like:

error: variables can be used directly in the `format!` string
   --> prost-derive/src/lib.rs:438:9
    |
438 | /         bail!(
439 | |             "invalid oneof {}: multiple variants have tag {}",
440 | |             ident,
441 | |             duplicate_tag
442 | |         );
    | |_________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args