Fix regression from #140393 for espidf / horizon / nuttx / vita
#140393 introduced changes to the layout of the `std::sys::process` code.
As a result, the Tier 3 ESP-IDF (and I suspect Horizon, Nuttx and Vita targets as well) no longer build.
A `pub use unsupported::output` is all that was missing - for the above OSes specifically. This explicit `pub use` is now necessary, because #140393 moved the `output` function to module-level, where it was previously part of `Command` and was thus re-exported automatically, as part of the `imp::Command` re-export further down the file containing the one-liner fix.
Note that - with the change introduced by #140393 - we **can't** anymore just do an unconditional `pub use imp::output` as this function simply does not exist anymore anywhere else but in the `unsupported` module.
r? `@joboet`
5152ab2f5e