Nix

Add nested commands #618

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Job	Run time
docs (ubuntu-20.04)	52s
flakes (ubuntu-20.04)	1m 21s
flakes (ubuntu-22.04)	1m 31s
flakes (macos-11)	4m 30s
build (ubuntu-20.04)	1m 22s
flakes (macos-12)	5m 8s
build (ubuntu-22.04)	1m 38s
build (macos-11)	5m 17s
build (macos-12)	8m 39s
	30m 18s