Downloads and store operations should not be performed as root

[dhess]

Issue description

I noticed on my new multi-user Nix install on macOS that fetches and store operations are performed as the root user. Builds are done by a nixbld user, as expected.

On macOS, at least, the store is g+w and the group is nixbld, so store operations should be done as a nixbld user for reasons of least privilege -- ditto for downloads.

On irc, @grahamc confirmed that this behavior is also manifest on NixOS. Presumably this behavior is also unwanted there.

Steps to reproduce

Use nix-env to build a package that hits the cache. While this is downloading and unpacking, use ps auxww | grep nix and observe that the downloads and store operations are performed by root.

[dhess]

Whoops, I guess this needs to be reported on nix.