Fix OS scripts for flakes

os/boot/default.nix

@@ -4,6 +4,7 @@
     grub = {
       enable = true;
       efiSupport = true;
+      efiInstallAsRemovable = !config.boot.loader.efi.canTouchEfiVariables;
       device = "nodev"; # Don't install on MBR
     };
   };

os/common.nix

@@ -21,6 +21,9 @@
     }];
   }];
 
+  # UPST disko --root-mountpoint doesn't work when using flake, workaround:
+  disko.rootMountPoint = "/mnt/nixos";
+
   environment.systemPackages = with pkgs; [
     wget
     kexec-tools