small `Check::instances()` usage cleanup #19767

Job	Run time
build_make (alpine:3.23)	5m 53s
build_make (ubuntu:25.10)	4m 50s
build_cmake (ubuntu:24.04, true, true)	5m 4s
build_cmake (ubuntu:25.10, true, true)	4m 58s
build_cmake (alpine:3.23, false, false)	4m 13s
build_make (ubuntu:24.04)	4m 12s
	29m 10s

Provide feedback