putenv() doesn't propagate into shell_exec()

### HHVM Version

```
HipHop VM 3.16.0-dev (rel)
Compiler: heads/master-0-ge97dcd8e6821b1f70173c0ba106e01c3f239d1e1
Repo schema: bbcd692673eb9a01ff839c7aefeff5b0cbcc89cb
```
### PHP Version

```
PHP 7.0.8-3ubuntu2 (cli) ( NTS )
Copyright (c) 1997-2016 The PHP Group
Zend Engine v3.0.0, Copyright (c) 1998-2016 Zend Technologies
with Zend OPcache v7.0.8-3ubuntu2, Copyright (c) 1999-2016, by Zend Technologies
with Xdebug v2.4.0, Copyright (c) 2002-2016, by Derick Rethans
```
### Standalone code, or other way to reproduce the problem

``` php
php test.php
string(3) "xyz"
```
### Actual result

```
> hhvm test.php
Notice: Undefined index: xyz in %s/echo.php on line 2
NULL
```