nit: fix image output to be a file, not an item inside a folder

hosts/modules/hal/pine64.nix

@@ -279,7 +279,7 @@ in
     sane.image.extraGPTPadding = 16 * 1024 * 1024;
     sane.image.firstPartGap = 0;
     sane.image.installBootloader = ''
-      dd if=${pkgs.tow-boot-pinephone}/Tow-Boot.noenv.bin of=$out/nixos.img bs=1024 seek=8 conv=notrunc
+      dd if=${pkgs.tow-boot-pinephone}/Tow-Boot.noenv.bin of=$out bs=1024 seek=8 conv=notrunc
     '';
 
     sane.programs.swaynotificationcenter.config = {

modules/image.nix

@@ -173,17 +173,20 @@ in
   in
   lib.mkIf cfg.enable
   {
-    system.build.img = (if cfg.installBootloader == null then
-      img
-    else pkgs.runCommand "nixos-with-bootloader" {} ''
-      cp -vR ${img} $out
-      chmod -R +w $out
-      ${cfg.installBootloader}
-    '') // {
+    system.build.img = pkgs.runCommandLocal "nixos-with-bootloader" {
       passthru = {
         inherit bootFsImg nixFsImg;
-        withoutBootloader = img;
+        withoutBootloader = img;  #< XXX: this derivation places the image at $out/nixos.img
       };
-    };
+    } (
+      if cfg.installBootloader == null then ''
+        ln -s ${img}/nixos.img $out
+      '' else ''
+        cp ${img}/nixos.img $out
+        chmod +w $out
+        ${cfg.installBootloader}
+        chmod -w $out
+      ''
+    );
   };
 }