CMake: fix use of system cereal #667

Job	Run time
Noetic / Debug	6m 20s
Noetic / Release	6m 18s
Jazzy / Debug	7m 56s
Jazzy / Release	8m 39s
Jazzy / Debug / Vectorization	8m 19s
Jazzy / Release / Vectorization	7m 56s
Iron / Debug	6m 50s
Iron / Release	7m 7s
Iron / Debug / Vectorization	7m 19s
Iron / Release / Vectorization	7m 24s
Humble / Debug / Vectorization	6m 41s
Humble / Release / Vectorization	6m 55s
Rolling / Debug / Vectorization	8m 1s
Rolling / Release / Vectorization	8m 10s
Rolling / TSID-Downstream	8m 37s
check-ci-linux-ros	4s
	1h 52m 36s

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