posix: env: use sizeof void ptr instead of char ptr ptr

Coverity regularly raised a CID with every release that it found
`sizeof(char **)` confusing and somehow nonportable.

With every release, it needed to be marked as a false positive.

Change `sizeof(char **)` to `sizeof(void *)` to silence the
CID warnings.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>