grid: do not hide `$USER` when running buildout
Hiding such environment variable prevent easy access to the user owning the software or the user owning the instance.
Some instances needs to have the actual username of the instance in their config files and some softwares needs access to the username of user building the software. This is - in theory - easy to do in zc.buildout thanks to https://collective.github.io/collective.recipe.grp/ , but the way slapos run buildout with some environment variables hidden made it not possible.
Fixes #20180402-716ACB