Add flake.nix and test on CI #75

Job	Run time
Generate GHC versions for nix flake build matrix	24s
Check Flake	23s
Haskell GHC 8.4.4 cabal	1m 42s
Haskell GHC 8.6.5 cabal	1m 44s
Haskell GHC 8.8.4 cabal	1m 54s
Haskell GHC 8.10.7 cabal	1m 39s
Haskell GHC 9.0.2 cabal	1m 46s
Haskell GHC 9.2.2 cabal	1m 43s
Haskell GHC 9.4.2 cabal	1m 55s
Haskell GHC 9.6.6 cabal	2m 10s
Haskell GHC 9.8.1 cabal	2m 14s
Haskell GHC 9.10.1 cabal	48s
Haskell GHC 9.12.1 cabal	41s
Nix Flake (ghc810, ubuntu-latest)	2m 16s
Nix Flake (ghc810, macos-latest)	22m 51s
Nix Flake (ghc90, ubuntu-latest)	2m 22s
Nix Flake (ghc90, macos-latest)	20m 50s
Nix Flake (ghc92, ubuntu-latest)	5m 53s
Nix Flake (ghc92, macos-latest)	26m 36s
Nix Flake (ghc94, ubuntu-latest)	2m 17s
Nix Flake (ghc94, macos-latest)	23m 7s
Nix Flake (ghc96, ubuntu-latest)	2m 41s
Nix Flake (ghc96, macos-latest)	3m 45s
Nix Flake (ghc98, ubuntu-latest)	2m 17s
Nix Flake (ghc98, macos-latest)	5m 5s
Nix Flake (ghc910, ubuntu-latest)	2m 23s
Nix Flake (ghc910, macos-latest)	3m 33s
	2h 24m 59s

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