add(chain): Add an NU6 network upgrade variant #1627

Job	Run time
Check deny.toml bans	26s
Check Cargo.lock is up to date	50s
Check deny.toml bans --features default-release-binaries	25s
Check deny.toml bans --all-features	27s
Install zebrad from lockfile without cache on ubuntu-latest	8m 46s
Check deny.toml sources	27s
Test stable on ubuntu-latest	32m 3s
Check deny.toml sources --features default-release-binaries	32s
Check for unused dependencies	12s
Test beta on ubuntu-latest	31m 36s
Check deny.toml sources --all-features	27s
Test stable on macos-latest	35m 30s
Test stable on windows-latest	1h 3m 17s
Test beta on windows-latest	59m 18s
Open or update issues for OS integration failures	0s
	3h 54m 16s

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