Add notes about alternative way to specify nightly with tool chains and how to set environment variables

[underscorefunk]

Setting environment variables or flags isn't well discussed anywhere. Lets explicitly state that they prefix the cargo or trunk commands. This would be a good spot to explain how the terminal works in that it's just executing code in the context of your shell. As a result, you can "set a variable and then run a command" as a one off.

I don't think most newbie programmers realize that when they're typing in terminal they're actually programming in a REPL of sorts.