Define LF_FILE_SEPARATOR in CMake #3853

Job	Run time
check-diff / check	17s
serialization / serialization-tests	10m 16s
py / py-tests / regular-tests (ubuntu-latest)	20m 17s
c / benchmarking / benchmark-tests	7m 4s
c / ccpp / regular-tests (ubuntu-latest)	24m 6s
c / default / regular-tests (ubuntu-latest)	26m 52s
cpp / cpp-benchmark-tests / benchmark-tests	11m 46s
c / verifier / run (ubuntu-latest)	7m 47s
cpp / cpp-tests / regular-tests (ubuntu-latest)	49m 19s
cpp / cpp-ros2-tests / cpp-ros2-tests	43m 45s
rs / rs-benchmark-tests / benchmark-tests	15m 42s
rs / rs-tests / regular-tests (ubuntu-latest, stable)	10m 35s
ts / benchmarking / benchmark-tests	5m 37s
ts / ts-tests / regular-tests (ubuntu-latest)	21m 59s
	4h 15m 22s

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