Drop nixpkgsLatest in favour of haskellNix/nixpkgs #6787

Job	Run time
Build specification using nix	2m 45s
Build & test (hydra-plutus)	2m 27s
Benchmarks (hydra-node, tx-cost, --output-directory $(pwd)/../benchmarks)	3m 15s
Build & test (hydra-tui)	4m 17s
Benchmarks (hydra-node, micro, -o $(pwd)/../benchmarks/ledger-bench.html)	2m 42s
Build haddock using nix	2m 9s
Build & test (hydra-node)	9m 23s
Benchmarks (hydra-cluster, bench-e2e, datasets datasets/1-node.json datasets/3-nodes.json --outpu...	5m 10s
Build & test (hydra-tx)	2m 59s
nix flake check	5m 21s
Build & test (hydra-cluster)	12m 57s
Compute cost differences	5m 21s
Publish benchmark results	7s
Documentation	3m 37s
Publish test results	12s
	1h 2m 42s

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