ignore nix profiles #1505

Job	Run time
quick-build (supported, ubuntu-latest)	4m 14s
quick-build (latest, ubuntu-latest)	3m 53s
quick-build (dev, ubuntu-latest)	4m 2s
opam-build (supported, ubuntu-latest)	5m 44s
opam-build (8.17, ubuntu-latest)	5m 46s
opam-build (latest, ubuntu-latest)	7m 3s
opam-build (dev, ubuntu-latest)	5m 9s
nix	4m 15s
build (supported)	3m 56s
build (latest)	3m 38s
build (dev, --warnings)	4m 22s
doc-dep-graphs	11m 54s
doc-coqdoc	1m 9s
coqchk (supported)	1m 55s
coqchk (latest)	2m 9s
coqchk (dev)	1m 53s
doc-alectryon	19m 45s
doc-timing	1m 14s
install (supported)	1m 20s
install (latest)	1m 23s
install (dev)	1m 26s
deploy-doc	0s
delete-artifacts	3s
	1h 36m 13s

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