Motivation for this change

Having environment.etc set using a list makes it very hard to override with mkForce
see: https://logs.nix.samueldr.com/nixos/2019-08-25#2523375;

Things done

Changed the type of environment.etc to attrsOf and changed every instance of it being set with a list in nixos

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @aanderse @cleverca22

I'm sure there are a lot of mistakes in what I've done, especially with mapAttrs'. Is there any way to go about checking this automatically?
Also I expect my commit names aren't very good or maybe they should be squashed, I'm happy to rebase if someone suggests something better