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.
Setting `outputSpecified` prevents `getOutput` from descending into
outputs, which don't have an overridden `outPath`.
Additionally, use `__spliced` to permit derivations to use the dummy as
an input.
This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
