ci: cache Nix store with cache-nix-action #2170

Job	Run time
Test memory (Nix)	5s
Build Linux static (Nix)	7s
Lint & check code style	5s
Test (Nix)	7s
Test PG 9.6 (Nix)	4s
Test PG 10 (Nix)	7s
Build Linux (Cabal, GHC 9.0.2)	25m 40s
Build MacOS (Nix)	8s
Test PG 11 (Nix)	4s
Build Linux (Cabal, GHC 9.2.4)	22m 39s
Test PG 12 (Nix)	4s
Build Linux (Stack)	3m 38s
Test PG 13 (Nix)	3s
Build MacOS (Stack)	3m 59s
Get FreeBSD build from CirrusCI	2m 5s
Build Windows (Stack)	6m 53s
Test PG 14 (Nix)	6s
Test PG 15 (Nix)	6s
Build aarch64 (Cabal, GHC ${{ matrix.ghc }})	0s
Test PG 16 (Nix)	3s
Prepare release	0s
Release on GitHub	0s
Release on Docker Hub	0s
Release Arm Builds on Docker Hub	0s
Remove copied files from server	0s
	1h 6m 3s

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