Add FLUTTER_ROOT environment variable

[FMorschel]

@pingbird I'm still not quite familiar with the project as a whole, but I think this would fix #147.

Could you please test this out, too? Thanks a lot!

[FMorschel]

Friendly ping @pingbird. I'd love to have this published as an official new version.

[hectorAguero]

This MR should fix dart run build_runner build not working when using Puro.

For now, not having that command working is a real pain point that makes using Puro a poor experience, because it's difficult to find out why it is not working.

As a temporary solution on my computer, I added the FLUTTER_ROOT variable to my .zshenv, so I can use flutter pub run command instead

export FLUTTER_ROOT="$HOME/.puro/envs/default/flutter" # Added manually for Puro (temporary)

And this alias:

alias drb="flutter pub run build_runner build"