fix: check if subsystem has builders properly

src/default.nix

@@ -242,7 +242,7 @@ in let
   findBuilder = dreamLock: let
     subsystem = dreamLock._generic.subsystem;
   in
-    if ! framework.buildersBySubsystem.${subsystem} ? default
+    if ! framework.buildersBySubsystem ? ${subsystem}
     then throw "Could not find any builder for subsystem '${subsystem}'"
     else framework.buildersBySubsystem.${subsystem}.default;
 

src/modules/builders/implementation.nix

@@ -9,6 +9,7 @@
     nodejs = "granular-nodejs";
     python = "simple-python";
     php = "granular-php";
+    haskell = "simple-haskell";
   };
   loader = b: b // {build = callPackageDream b.build {};};
   funcs = config.functions.subsystem-loading;