Bump nixpkgs to around nixos-24.11 and ghc to 9.6 #305

Job	Run time
nix / ubuntu-latest	4m 7s
nix flakes / ubuntu-latest	3m 56s
cabal / ghc-9.4.8 / ubuntu-20.04	3m 21s
nix simple cabal shell.nix / ubuntu-latest	13m 0s
cabal / ghc-9.6.6 / ubuntu-20.04	3m 33s
stack --stack-yaml ./stack.yaml / ubuntu-20.04	11m 35s
cabal / ghc-9.8.4 / ubuntu-20.04	2m 33s
cabal / ghc-9.4.8 / ubuntu-22.04	3m 24s
cabal / ghc-9.6.6 / ubuntu-22.04	3m 49s
stack --stack-yaml ./stack.yaml / ubuntu-22.04	11m 34s
cabal / ghc-9.8.4 / ubuntu-22.04	2m 10s
stack --stack-yaml ./stack.yaml / ubuntu-24.04	11m 18s
cabal / ghc-9.4.8 / ubuntu-24.04	3m 30s
stack --stack-yaml ./stack-nightly.yaml / ubuntu-20.04	11m 35s
cabal / ghc-9.6.6 / ubuntu-24.04	3m 40s
stack --stack-yaml ./stack-nightly.yaml / ubuntu-22.04	11m 34s
cabal / ghc-9.8.4 / ubuntu-24.04	2m 26s
stack --stack-yaml ./stack-nightly.yaml / ubuntu-24.04	11m 35s
	1h 58m 40s

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