ci: merge cached Nix stores #2351

Job	Run time
Build Linux (Cabal, GHC 9.0.2)	1m 33s
Build aarch64 (Cabal, GHC ${{ matrix.ghc }})	0s
Build MacOS (Nix)	7m 50s
Build Linux (Cabal, GHC 9.2.4)	1m 29s
Build Linux (Stack)	39s
Get FreeBSD build from CirrusCI	2m 21s
Prepopulate Nix cache for Linux runners	1m 18s
Build MacOS (Stack)	4m 3s
Build Windows (Stack)	3m 37s
Test memory (Nix)	1m 53s
Lint & check code style	1m 35s
Build Linux static (Nix)	1m 19s
Test (Nix)	5m 42s
Test PG 9.6 (Nix)	4m 22s
Test PG 10 (Nix)	4m 0s
Test PG 11 (Nix)	4m 11s
Test PG 12 (Nix)	4m 16s
Test PG 13 (Nix)	3m 57s
Test PG 14 (Nix)	3m 54s
Test PG 15 (Nix)	3m 52s
Test PG 16 (Nix)	4m 0s
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 5m 51s

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