Use composite github action to deduplicate nix and cachix setup #6902

Job	Run time
Build haddock using nix	1m 1s
Build & test (hydra-plutus)	1m 23s
Build & test (hydra-tui)	3m 7s
nix flake check	4m 29s
Build & test (hydra-node)	7m 51s
Benchmarks (hydra-node, tx-cost, --output-directory $(pwd)/../benchmarks)	2m 13s
Build & test (hydra-tx)	1m 54s
Benchmarks (hydra-node, micro, -o $(pwd)/../benchmarks/ledger-bench.html)	1m 36s
Build & test (hydra-cluster)	11m 43s
Benchmarks (hydra-cluster, bench-e2e, datasets datasets/1-node.json datasets/3-nodes.json --outpu...	4m 7s
Build specification using nix	1m 34s
Compute cost differences	3m 13s
Publish benchmark results	7s
Publish test results	17s
Documentation	21s
	44m 56s

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