Run VirtualBox as the configured user when installed as a Linux service

On Linux, GitLab Runner runs as root when installed as a service, which in turn results in VBoxManage being run as root when using the VirtualBox executor, ignoring the user specified when the service is installed. This commit modifies the behavior of the VirtualBox executor to run VBoxManage commands as the user specified during service installation (if one was specified) by starting it with the "sudo" command instead of directly.

Fixes #1121