nix flake pin: Mark the resulting registry entry as exact #439

Job	Run time
vm_tests	18m 7s
tests on ubuntu	14m 47s
Check Docker secrets present for installer tests	1s
eval	53s
tests on macos	14m 13s
installer test on ubuntu	1m 35s
installer test on macos	1m 33s
flake_regressions	12m 20s
docker_push_image	0s
	1h 3m 29s

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