Ben Gamari authored Dec 23, 2021 and Matthew Pickering committed Jan 31, 2022

Previously we would unconditionally provide a declaration for `environ`,
even if `<unistd.h>` already provided one. This would result in
`-Werror` builds failing on some platforms.

Also `#include <unistd.h>` to ensure that the declaration is visible.

Fixes #20861.

(cherry picked from commit 247cd336)