See if Debian package can automatically modify $PATH to include Yarn global directory

**Do you want to request a *feature* or report a *bug*?**
Feature

**What is the current behavior?**
People that install Yarn need to know to manually add its global executable directory to the $PATH

**What is the expected behavior?**
See if we can automate this for the Debian package, somehow. We already do it for the Windows installer.