packages (i.e., the packages that should appear in the user's $PATH,
and so on). Based on this list, the script nix-populate creates a
hierarchy of symlinks to the relevant files in those packages (e.g.,
for pkg/bin and pkg/lib).
A nice property of nix-populate is that on each run it creates a
*new* tree, rather than updating the old one. It then atomically
switches over to the new tree. This allows atomic upgrades or
rollbacks on the set of activated packages.
