Commit 8109d8a set the `StateDirectory=` option of the systemd service
configuration to the value of `cfg.workDir` which is wrong, according
to dasJ [1]. This commit resolves this issue by stripping the
`/var/lib/` prefix from `cfg.workDir`.
[1] https://github.com/NixOS/nixpkgs/pull/172824#issuecomment-1130350412