They would have to be resealable aluminum cans. Apparently those exist though.
Jugs are a no-go. The point of 2.0 is that it's portable, already measured out for you so it's easy to keep track of calories, and ready to drink without pouring it into a container. Jugs are none of those things. There's no reason in principle Soylent couldn't come in jugs also, but it wouldn't be a replacement for the Soylent 2.0 packaging form factor.
Is aluminum really more environmentally friendly than plastic? Sure, plastic might fill up landfills instead of getting recycled like aluminum, but landfill space is practically unlimited and the land from landfills can eventually be reclaimed, e.g. turned into nice parks. I would like to look at the production side and see whether plastic or aluminum pollutes more.
This isn't quite the same, but Slate ran an article that weighed the environmental impact of plastic wrap versus aluminum foil:
I don't think optimizing packaging is a bad idea at all, but it's important to put it in perspective. Worrying about Soylent's packaging is a bit like worrying whether the statements for the monthly payments on your electric car are paperless. The environmental impact of plant-based food versus animal-based food is like solar power vs. coal — maybe not exactly, but it's at that kind of scale.