fix: Use --nvidia when creating distrobox containers + Switch to new justfile system

[KyleGospo]

Depends on the following PR landing:
ublue-os/config#118

[xynydev]

Why the removal of set allow-duplicate-recipes, doesn't that break things? Also, a corresponding PR for startingpoint needs to be done before merging this: (https://github.com/ublue-os/startingpoint/blob/template/config/files/usr/share/ublue-os/just/custom.just). I can also PR startingpoint, once I know what the correct course of action is. Should custom.just also be named with a number prefix? And with this new system, does 100-bling.just have to be included in it or is it included automatically?

Edit: Answers:

• rename custom.just to 60-custom.just
• change include to include 100-bling.just (in startingpoint after this PR is merged)