[Process] Add ProcessBuilder::setPrefix method

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT

This introduce a new feature for the `ProcessBuilder` that allows to prefix all generated `Process` commands with a custom value.
This is mostly useful when dealing with binary drivers. 

Use case : 

```php
use Symfony\Component\Process\ExecutableFinder;
use Symfony\Component\Process\ProcessBuilder;

class PhpDriver
{
    private $pb;

    public function __construct(ProcessBuilder $pb)
    {
        $this->pb = $pb;
    }

    public function getInfos()
    {
        return $this->execute('-i');
    }

    public function getVersion()
    {
        return $this->execute('-v');
    }

    private function execute(array $arguments)
    {
        return $this
            ->pb
            ->setArguments($arguments)
            ->createProcess()
            ->run()
            ->getOutput();
    }
}

$finder = new ExecutableFinder();

$pb = new ProcessBuilder();
$pb->setPrefix($finder->find('php'));

$driver = new PhpDriver($pb);
$driver->getVersion();
$driver->getInfos();
```