adapt to coq/coq#19872 (#150) #58

Job	Run time
build (coqorg/coq:8.9)	5m 16s
build (coqorg/coq:8.11)	5m 17s
build (coqorg/coq:8.12-ocaml-4.10-flambda)	6m 41s
build (coqorg/coq:8.13)	11m 24s
build (coqorg/coq:8.14)	10m 16s
build (coqorg/coq:8.15)	10m 42s
build (coqorg/coq:8.16)	10m 33s
build (coqorg/coq:8.17)	10m 37s
build (coqorg/coq:8.18)	10m 22s
build (coqorg/coq:8.19)	1h 4m 56s
build (coqorg/coq:8.20)	11m 7s
build (coqorg/coq:dev)	8m 56s
	2h 46m 7s

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