Make `nix flake metadata|update|lock` lazy #386

Job	Run time
tests on ubuntu	13m 22s
Check Docker secrets present for installer tests	1s
eval	48s
tests on macos	13m 10s
vm_tests	9m 31s
flake_regressions	0s
installer test on ubuntu	32s
docker_push_image	0s
installer test on macos	1m 32s
	38m 56s

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