io: add track_caller caller to no-rt io driver (#4852)
io: add track_caller caller to no-rt io driver (#4852)

This change adds a case that was missing from the original PR, #4793.

The `io::driver::Handle::current` function was only covered by
`#[track_caller]` in the case that the `rt` feature  is enabled, however
it was missing in the case that the `rt` feture isn't enabled (in which
case a panic would be more common).

This particular case cannot be tested in the tokio tests as they always
run with all features enabled.

Refs: #4413
tokio-rs/tokio,tokio-rs/tokio