[Suggestion] init_hooks run as user

**Is your feature request related to a problem? Please describe.**
I'm always frustrated when I have to run `su $USER -- cmd` in init_hooks in distrobox assemble

**Describe the solution you'd like**
Obviously a conventional way to execute init_hooks as the distrobox's user in the container obviously through distrobox-init

**Describe alternatives you've considered**
As mentioned `su $USER -- cmd` in init hook but it's kinda depend on weird behavior like the `$USER` variable expanding in v1 but I remember most of the variable expansion doesn't on the v2 distrobox.ini file