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.
systemd rejects the service unit due to whitespace in the environment
variable assignment, pointing to the repo path, being invalid for
systemd's unit format.
See https://github.com/nix-community/home-manager/issues/6023 for
details.
The git-sync variable should also be escaped due to similar issues
with e.g. local git urls.