Compatibility: Only use `sycl::half` if SYCL_CTS_ENABLE_HALF_TESTS is set #2897

Job	Run time
check-secrets	1s
check-clang-format	4s
build-common-base-image	0s
build-image-for-sycl-impl	0s
compile-cts (dpcpp, ca955e538171cb7b7eb07734dd5c2b958c84901c)	1h 58m 38s
compile-cts (adaptivecpp, 061e2d6ffe1084021d99f22ac1f16e28c6dab899)	2m 37s
cts-compiles-for-all-implementations	1s
	2h 1m 21s

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