[Process] add a method to the ProcessBuilder to automatically prepend the executed script with "exec"

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | symfony/symfony-docs#3999

This pull requests addresses the issue raised by @romainneutron in #5759. It eases the process of creating a ``Process`` whose script should be prepended by `exec`.
you cannot use ``exec`` for multiple commands.
But actually, I'm not sure the ProcessBuilder allows building such commands though. It needs to be checked.
the case of sigchild is an issue though.

I don't know about such a command for windows. @romainneutron do you have an idea ?