shell.nix: Use default.nix as source of truth

shell.nix

@@ -1,6 +1,18 @@
-with import <nixpkgs> {};
+{ pkgs ? import <nixpkgs> {} }:
-mkShell {
+
-  nativeBuildInputs = [ grub2_efi inkscape imagemagick ];
+let
-  dejavu = dejavu_fonts;
+  default = import ./. { inherit pkgs; };
-  __strictDeps = true;
+in
-}
+pkgs.callPackage (
+  { mkShell
+  , dejavu_fonts
+  }:
+  mkShell {
+    nativeBuildInputs = []
+      ++ default.bootloader.nativeBuildInputs
+      ++ default.icons.nativeBuildInputs
+    ;
+    dejavu = dejavu_fonts;
+    __strictDeps = true;
+  }
+) {}