Add a Nix flake #2160

Job	Run time
Generate crates matrix	16s
Check crates matrix	2s
Build zebrad crate	36m 37s
Build zebra-utils crate	16m 49s
Build zebra-test crate	1m 33s
Build zebra-state crate	22m 43s
Build zebra-script crate	3m 11s
Build zebra-scan crate	26m 7s
Build zebra-rpc crate	36m 25s
Build zebra-node-services crate	3m 15s
Build zebra-network crate	6m 49s
Build zebra-grpc crate	14m 32s
Build zebra-consensus crate	18m 0s
Build zebra-chain crate	4m 51s
Build tower-fallback crate	1m 8s
Build tower-batch-control crate	1m 26s
Open or update issues for building crates individually failures	0s
	3h 13m 44s

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