nix: update flake and rocq #3937

Job	Run time
opam-build (supported, ubuntu-latest)	4m 44s
opam-build (latest, ubuntu-latest)	4m 44s
quick-build (supported, ubuntu-latest)	3m 30s
nix	2m 14s
quick-build (dev, ubuntu-latest)	3m 23s
quick-build (latest, ubuntu-latest)	3m 28s
build (supported)	3m 36s
opam-build (dev, ubuntu-latest)	4m 18s
build (dev, --warnings)	3m 56s
build (latest)	3m 35s
doc-timing	1m 56s
doc-coqdoc	1m 24s
doc-dep-graphs	8m 20s
install (dev)	1m 53s
install (supported)	1m 51s
coqchk (dev)	1m 59s
doc-alectryon	15m 17s
coqchk (latest)	1m 58s
install (latest)	1m 37s
coqchk (supported)	2m 30s
delete-artifacts	8s
deploy-doc	0s
	1h 16m 21s

Provide feedback