syscall: fix Setenv for plan 9

envi needs to be updated during Setenv so the key can be correctly deleted later with Unsetenv.

Update issue 8849.

LGTM=0intro
R=bradfitz, 0intro
CC=golang-codereviews
https://codereview.appspot.com/149300046

Committer: David du Colombier <0intro@gmail.com>