optimized `Library::platform_type()`a bit #19160

Job	Run time
build_cmake (ubuntu:24.04, true, true)	5m 28s
build_make (ubuntu:25.10)	5m 32s
build_cmake (ubuntu:25.10, true, true)	5m 35s
build_make (ubuntu:24.04)	4m 47s
build_make (alpine:3.23)	6m 4s
build_cmake (alpine:3.23, false, false)	4m 33s
	31m 59s

Provide feedback