Fix nix flake check warnings

flake.nix

@@ -1,12 +1,7 @@
 {
   description = "Alternative Haskell Infrastructure for Nixpkgs";
 
-  outputs = { self }: {
-    # Using the eval-on-build version here as the plan is that
-    # `builtins.currentSystem` will not be supported in flakes.
-    # https://github.com/NixOS/rfcs/pull/49/files#diff-a5a138ca225433534de8d260f225fe31R429
-    overlay = (self.overlays {sourcesOverride = self.sources;}).combined-eval-on-build;
-    overlays = import ./overlays;
+  outputs = { self }: let
     config = import ./config.nix;
     # We can't import ./nix/sources.nix directly, because that uses nixpkgs to fetch by default,
     # and importing nixpkgs without specifying localSystem doesn't work on flakes.
@@ -21,6 +16,12 @@
       inherit (self) config;
       overlays = [ self.overlay ];
     };
+  in {
+    # Using the eval-on-build version here as the plan is that
+    # `builtins.currentSystem` will not be supported in flakes.
+    # https://github.com/NixOS/rfcs/pull/49/files#diff-a5a138ca225433534de8d260f225fe31R429
+    overlay = self.overlays.combined-eval-on-build;
+    overlays = import ./overlays { sourcesOverride = self.sources; };
     legacyPackages = let
       genAttrs = lst: f:
         builtins.listToAttrs (map (name: {