Compatibility: Only use `sycl::half` if SYCL_CTS_ENABLE_HALF_TESTS is set #2873

Job	Run time
check-secrets	1s
check-clang-format	13s
build-common-base-image	0s
build-image-for-sycl-impl	0s
compile-cts (dpcpp, 6456fe89646deb8bf30c0eb32827a62ff6e58ffb)	29m 57s
compile-cts (adaptivecpp, 061e2d6ffe1084021d99f22ac1f16e28c6dab899)	2m 46s
cts-compiles-for-all-implementations	1s
	32m 58s

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