fix proper environment on aptly execute

I've been testing the cookbook with debian wheezy and run into the problem, that the "seed aptly db" command fails because it misses the HOME and USER environment vars mentioned here: https://tickets.opscode.com/browse/CHEF-2288

This fix is however not a proper solution as there are several more execute commands, half of them using the environment, the other half do not.

I think the problem could be worked around by using the bash resource instead of using execute + environment setup or by using an aptly specific wrapper around execute, but I'm not sure how to do it properly. I suggest to simply use bash everywhere in the cookbook.