core/service: fix deserialization of non-absolute commands

We'd fail with:
Apr 23 10:58:26 systemd[1]: Deserializing state...
Apr 23 10:58:26 systemd[1]: testsuite-01.service: Failed to parse serialized command "ExecStart 0 sh "sh" "-e" "-x" "-c" "systemctl --state=failed --no-legend --no-pager >/failed ; systemctl daemon-reload ; echo OK >/testok"": Invalid argument
Apr 23 10:58:26 systemd[1]: testsuite-01.service: Reinstalled deserialized job testsuite-01.service/start as 209

This was missed in 5008da1e, and apparently nobody noticed until now :(

(cherry picked from commit 1a128a46)
(cherry picked from commit 27606d54)
(cherry picked from commit 3b7a2cf0)