With ProtectSystem=strict (added in the previous commit), / is read-only
inside hive-priv. nix creates a temp result symlink in its cwd; without
WorkingDirectory that cwd is / (systemd default), causing:
error: creating symlink /.tmp.tmp-...: Read-only file system
Point WorkingDirectory at the StateDirectory (/var/lib/hive-priv) which is
always writable under strict. Nix drops its temp symlink there instead.
9aa624d310