nix: Reduce number of nixpkgs variants

Now the `applyPatches` (derivation-creating) function can be supplied
from a pre-instantiated nixpkgs derivation, or from
`nixpkgs.legacyPackages` if `nixpkgs` is a flake, or from an import of
the unpatches `nixpkgs` otherwise.

The previous code would instantiate a version of nixpkgs just for the
call to `applyPatches`.

default.nix

@@ -11,6 +11,14 @@
     url = "https://github.com/nixos/nixpkgs/archive/${nixpkgsVersion.rev}.tar.gz";
     sha256 = nixpkgsVersion.tarballHash;
   }
+, applyPatches ? (
+    # If nixpkgs is a flake
+    nixpkgs.legacyPackages.${system}
+      or
+      # If nixpkgs is a derivation
+      import nixpkgs
+      { inherit system; }
+  ).applyPatches
 }:
 
 let
@@ -26,7 +34,8 @@ let
       (pkgs.gitignoreSource ./.)
       [ ".cabal" ".hs" ".lhs" "LICENSE" ];
 
-  nixpkgs-patched = (import nixpkgs { inherit overlays system; }).applyPatches {
+  # Workaround for https://github.com/NixOS/nixpkgs/issues/286285
+  nixpkgs-patched = applyPatches {
     name = "nixpkgs-patched";
     src = nixpkgs;
     patches = [ nix/split-sections-cross.patch ];