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.
Fixes tests that are affected by <https://github.com/NixOS/nixpkgs/pull/333744>.
DRY_RUN_CMD
DRY_RUN_NULL
As a replacement, this adds the `run` helper function.
systemd now depends on `gnupg.override`, so we need a stub for systemd too.
PR #810
