update nix flake #1504

Job	Run time
nix	3m 54s
quick-build (supported, ubuntu-latest)	4m 10s
quick-build (latest, ubuntu-latest)	3m 15s
quick-build (dev, ubuntu-latest)	3m 16s
build (supported)	3m 57s
build (latest)	3m 36s
build (dev, --warnings)	3m 49s
opam-build (supported, ubuntu-latest)	5m 47s
opam-build (8.17, ubuntu-latest)	6m 56s
opam-build (latest, ubuntu-latest)	5m 29s
opam-build (dev, ubuntu-latest)	5m 17s
doc-coqdoc	1m 9s
doc-alectryon	20m 7s
doc-dep-graphs	10m 9s
coqchk (supported)	1m 50s
coqchk (latest)	1m 43s
coqchk (dev)	1m 39s
doc-timing	1m 13s
install (supported)	1m 22s
install (latest)	1m 22s
install (dev)	1m 22s
deploy-doc	0s
delete-artifacts	6s
	1h 31m 28s

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