refactor: clarify iterating `systems` vs `block_systems` #1381

Job	Run time
report-coverage (3.10, macos-latest)	4m 7s
build (3.10, ubuntu-latest)	3m 28s
build (3.10, macos-latest)	2m 23s
build (3.11, ubuntu-latest)	3m 19s
build (3.11, macos-latest)	1m 30s
	14m 47s

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