Speed up Colimit_Flattening #1659

Job	Run time
nix	3m 33s
quick-build (supported, ubuntu-latest)	3m 20s
build (supported)	3m 46s
opam-build (supported, ubuntu-latest)	6m 47s
quick-build (latest, ubuntu-latest)	3m 54s
build (latest)	5m 28s
opam-build (8.17, ubuntu-latest)	6m 44s
quick-build (dev, ubuntu-latest)	4m 47s
build (dev, --warnings)	4m 21s
opam-build (latest, ubuntu-latest)	6m 24s
opam-build (dev, ubuntu-latest)	7m 19s
doc-coqdoc	1m 22s
coqchk (supported)	2m 14s
doc-alectryon	18m 0s
doc-dep-graphs	9m 47s
coqchk (latest)	2m 5s
coqchk (dev)	2m 12s
doc-timing	1m 14s
install (supported)	1m 12s
install (latest)	1m 15s
install (dev)	1m 15s
deploy-doc	0s
delete-artifacts	7s
	1h 37m 6s

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