Rollup merge of #140143 - thaliaarchi:move-env-pal, r=joboet

Move `sys::pal::os::Env` into `sys::env`

Although `Env` (as `Vars`), `Args`, path functions, and OS constants are publicly exposed via `std::env`, their implementations are each self-contained. Keep them separate in `std::sys` and make a new module, `sys::env`, for `Env`.

Also fix `unsafe_op_in_unsafe_fn` for Unix and update the `!DynSend` and `!DynSync` impls which had grown out of sync with the platforms (see #48005 for discussion on that).

r? joboet

Tracked in #117276.

Author: matthiaskrgr