use GHC 9.12 in Nix Flake

Author: peterbecich

flake.nix

@@ -42,7 +42,7 @@
         };
         packages.default = config.packages.hackage-server;
         haskellProjects.default = {
-          basePackages = pkgs.haskell.packages.ghc910;
+          basePackages = pkgs.haskell.packages.ghc912;
           settings = {
             hackage-server.check = false;