Auto merge of #29305 - alexcrichton:bad-getenv, r=brson

As discovered in #29298, `env::set_var("", "")` will panic, but it turns out
that it *also* deadlocks on Unix systems. This happens because if a panic
happens while holding the environment lock, we then go try to read
RUST_BACKTRACE, grabbing the environment lock, causing a deadlock.

Specifically, the changes made here are:

* The environment lock is pushed into `std::sys` instead of `std::env`. This
  also only puts it in the Unix implementation, not Windows where the functions
  are already threadsafe.
* The `std::sys` implementation now returns `io::Result` so panics are
  explicitly at the `std::env` level.

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