Add "fake" `Env.set_var` function

`Cmd.env` is what should be used so maybe we should guide people to that just like [`Str.len`](https://www.roc-lang.org/builtins/Str#len). Proposal:
```roc
set_var : (Str, Str) -> [UseCmdDotEnvInstead(Str)]
set_var = |_|
    UseCmdDotEnvInstead("Setting an environment variable first for a future command can make it hard to know which env vars affect what, check out `Cmd.Env` instead.") 
```