Latest clippy gained a warning lint about this:

```
error: variables can be used directly in the `format!` string
   --> zbus_macros/src/proxy.rs:199:30
    |
199 |             .or_else(|| Some(format!("/org/freedesktop/{}", ident)));
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
help: change this to
    |
199 -             .or_else(|| Some(format!("/org/freedesktop/{}", ident)));
199 +             .or_else(|| Some(format!("/org/freedesktop/{ident}")));
    |
```

Luckily, clippy can fix it automatically for us as well.