CMake: fix use of system cereal #1152

Job	Run time
[conda:ubuntu-latest:Release:c++17]	22m 0s
[conda:ubuntu-latest:Release:c++20]	23m 45s
[conda:macos-latest:Release:c++17]	15m 56s
[conda:macos-latest:Release:c++20]	14m 13s
[conda:windows-2019:Release:c++17]	24m 7s
[conda:windows-2019:Release:c++20]	25m 27s
[conda:windows-latest:Release:c++17]	30m 7s
[conda:windows-latest:Release:c++20]	24m 43s
[conda:macos-14:Release:c++17]	4m 23s
[conda:macos-14:Release:c++20]	4m 18s
[conda:ubuntu-latest:Debug:c++17]	4m 28s
[conda:ubuntu-latest:Debug:c++20]	4m 36s
[conda:windows-2019:Debug:c++17]	24m 23s
[conda:windows-2019:Debug:c++20]	29m 4s
[conda:windows-latest:Debug:c++17]	23m 38s
[conda:windows-latest:Debug:c++20]	26m 37s
[conda:macos-14:Debug:c++17]	5m 28s
[conda:macos-14:Debug:c++20]	4m 57s
[conda:macos-latest:Release:c++14]	15m 32s
[conda:macos-latest:Debug:c++17]	17m 0s
[conda:macos-latest:Debug:c++20]	39m 39s
check-ci-linux-osx-win-conda	4s
	6h 24m 25s

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