Add nix service definition for codex #4145

Job	Run time
coverage	18m 14s
matrix	16s
build / linux-unittest-amd64-pinned	16m 47s
build / linux-contract-amd64-pinned	10m 34s
build / linux-integration-amd64-pinned	1h 14m 45s
build / linux-tools-amd64-pinned	10m 28s
build / macos-unittest-amd64-pinned	26m 41s
build / macos-contract-amd64-pinned	15m 53s
build / macos-integration-amd64-pinned	1h 31m 2s
build / macos-tools-amd64-pinned	16m 31s
build / windows-unittest-amd64-pinned	35m 4s
build / windows-contract-amd64-pinned	21m 13s
build / windows-integration-amd64-pinned	1h 30m 57s
build / windows-tools-amd64-pinned	23m 14s
build / status	1s
	7h 31m 40s

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