syscall: fix Setenv for plan 9

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

Update #8849.

LGTM=0intro
R=bradfitz, 0intro
CC=golang-codereviews
https://golang.org/cl/149300046

src/syscall/env_plan9.go

@@ -128,6 +128,7 @@ func Setenv(key, value string) error {
 	}
 	env[key] = value
 	envs = append(envs, key+"="+value)
+	envi[key] = len(envs) - 1
 	return nil
 }