Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
The `exec` command does not do any shell parsing and does not
understand the `&&` which is how the extraCommands are added after
dbus activation. There doesn't seem to be a reason for `exec` here
anyway so just remove it and allow shell parsing.
