Nix-based CI

This leans on @mpickering's ghc-artefact-nix and head.hackage to provide testing of Hackage packages using GHC binary distributions.

patches/aeson-1.4.2.0.patch

+diff --git a/aeson.cabal b/aeson.cabal
+index 6193894..9307e9f 100644
+--- a/aeson.cabal
++++ b/aeson.cabal
+@@ -107,7 +107,7 @@ library
+     containers       >= 0.4.2.1 && < 0.7,
+     deepseq          >= 1.3.0.0 && < 1.5,
+     ghc-prim         >= 0.2     && < 0.6,
+-    template-haskell >= 2.7.0.0 && < 2.15,
++    template-haskell >= 2.7.0.0 && < 2.16,
+     text             >= 1.2.3.0 && < 1.3,
+     time             >= 1.4     && < 1.9
+