flake.nix: Pass haskellNix and isInFlake

Refactor flake.nix to pass `haskellNix` and `isInFlake` to
`default.nix` and drop haskellNix.config

* Depending on the value of `isInFlake`, nix/default.nix picks either
haskellNix.overlay or haskellNix.overlays.
* Drop haskellNix.config which doesn't seem relevant for plutus (plutus
doesn't target windows and doesn't use nixpkgs-20.03)

default.nix

@@ -11,17 +11,14 @@
 , config ? { allowUnfreePredicate = (import ./nix/lib/unfree.nix).unfreePredicate; }
 , sourcesOverride ? { }
 , sources ? import ./nix/sources.nix { inherit system; } // sourcesOverride
+, isInFlake ? false
 , haskellNix ? import sources."haskell.nix" {
     sourcesOverride = {
       hackage = sources."hackage.nix";
       stackage = sources."stackage.nix";
     };
   }
-  # haskell-nix has to be used differently in flakes/no-flakes scenarios:
-  # - When imported from flakes, 'haskellNix.overlay' needs to be passed here.
-  # - When imported from default.nix without flakes, default to haskellNix.overlays
-, haskellNixOverlays ? haskellNix.overlays
-, packages ? import ./nix { inherit system sources crossSystem config sourcesOverride haskellNix haskellNixOverlays checkMaterialization enableHaskellProfiling; }
+, packages ? import ./nix { inherit system sources crossSystem config sourcesOverride haskellNix isInFlake checkMaterialization enableHaskellProfiling; }
   # An explicit git rev to use, passed when we are in Hydra
   # Whether to check that the pinned shas for haskell.nix are correct. We want this to be
   # false, generally, since it does more work, but we set it to true in the CI

flake.nix

@@ -12,7 +12,8 @@
       let
         topLevel = import ./. {
           inherit system;
-          haskellNixOverlays = [ haskell-nix.overlay ];
+          isInFlake = true;
+          haskellNix = haskell-nix;
         };
 
         inherit (topLevel) pkgs plutus;

nix/default.nix

@@ -4,8 +4,8 @@
 , overlays ? [ ]
 , sourcesOverride ? { }
 , sources
+, isInFlake
 , haskellNix
-, haskellNixOverlays
 , checkMaterialization ? false
 , enableHaskellProfiling ? false
 }:
@@ -18,12 +18,16 @@ let
       (import ./overlays/r.nix)
     ];
 
+  # haskell-nix has to be used differently in flakes/no-flakes scenarios:
+  # - When imported from flakes, 'haskellNix.overlay' needs to be passed here.
+  # - When imported from default.nix without flakes, default to haskellNix.overlays
+  haskellNixOverlays = if isInFlake then [ haskellNix.overlay ] else haskellNix.overlays;
+
   extraOverlays = haskellNixOverlays ++ ownOverlays;
 
   pkgs = import sources.nixpkgs {
-    inherit system crossSystem;
+    inherit config system crossSystem;
     overlays = extraOverlays ++ overlays;
-    config = haskellNix.config // config;
   };
 
   plutus = import ./pkgs { inherit pkgs checkMaterialization enableHaskellProfiling sources; };