bpo-37931: Fix crash on OSX re-initializing os.environ (GH-15428)

bpo-37931: Fix crash on OSX re-initializing os.environ (GH-15428)

On most platforms, the `environ` symbol is accessible everywhere.

In a dylib on OSX, it's not easily accessible, you need to find it with
_NSGetEnviron.

The code was caching the *value* of environ. But a setenv() can change the value,
leaving garbage at the old value. Fix: don't cache the value of environ, just
read it every time.
(cherry picked from commit 723f71abf7ab0a7be394f9f7b2daa9ecdf6fb1eb)

Co-authored-by: Benoit Hudson <benoit@imgspc.com>