optimized `Library::platform_type()`a bit (#8380) #19208

Job	Run time
build_make (ubuntu:24.04)	4m 24s
build_cmake (alpine:3.23, false, false)	4m 5s
build_make (alpine:3.23)	6m 1s
build_make (ubuntu:25.10)	5m 6s
build_cmake (ubuntu:24.04, true, true)	5m 38s
build_cmake (ubuntu:25.10, true, true)	5m 41s
	30m 55s

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