build: use the system provided LuaJIT if found #9588

Job	Run time
run-ubuntu-unit-tests (-DFLB_JEMALLOC=On, gcc)	12m 59s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=On, clang)	13m 21s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=Off, gcc)	13m 0s
run-ubuntu-unit-tests (-DFLB_JEMALLOC=Off, clang)	12m 20s
run-ubuntu-unit-tests (-DFLB_SMALL=On, gcc)	12m 42s
run-ubuntu-unit-tests (-DFLB_SMALL=On, clang)	13m 23s
run-ubuntu-unit-tests (-DSANITIZE_ADDRESS=On, gcc)	12m 52s
run-ubuntu-unit-tests (-DSANITIZE_ADDRESS=On, clang)	13m 10s
run-ubuntu-unit-tests (-DSANITIZE_UNDEFINED=On, gcc)	12m 37s
run-ubuntu-unit-tests (-DSANITIZE_UNDEFINED=On, clang)	12m 47s
run-ubuntu-unit-tests (-DFLB_COVERAGE=On, gcc)	45m 4s
run-ubuntu-unit-tests (-DFLB_SANITIZE_MEMORY=On, gcc)	12m 28s
run-ubuntu-unit-tests (-DFLB_SANITIZE_MEMORY=On, clang)	12m 39s
run-ubuntu-unit-tests (-DFLB_SANITIZE_THREAD=On, gcc)	12m 19s
run-ubuntu-unit-tests (-DFLB_SANITIZE_THREAD=On, clang)	12m 14s
run-macos-unit-tests (-DFLB_JEMALLOC=Off)	12m 1s
run-macos-unit-tests (-DFLB_SANITIZE_MEMORY=On)	12m 8s
run-macos-unit-tests (-DFLB_SANITIZE_THREAD=On)	12m 21s
Unit tests (matrix)	1s
	4h 20m 26s

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