Add a Nix flake #2243

Job	Run time
Check Cargo.lock is up to date	8m 23s
Install zebrad from lockfile without cache on ubuntu-latest	8m 42s
Test stable on ubuntu-latest	40m 20s
Test beta on ubuntu-latest	39m 49s
Test stable on macos-latest	43m 31s
Test stable on windows-latest	1h 1m 4s
Test beta on windows-latest	1h 4m 26s
Check deny.toml bans	43s
Check for unused dependencies	15s
Check deny.toml bans --features default-release-binaries	43s
Check deny.toml bans --all-features	43s
Check deny.toml sources	43s
Check deny.toml sources --features default-release-binaries	45s
Check deny.toml sources --all-features	44s
Open or update issues for OS integration failures	0s
	4h 30m 51s

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