fix GRUB missing in Windows/Linux dual-boot

Open a command prompt with elevated rights (run as admin) and run:

bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi

To restore to previous settings run:

bcdedit /deletevalue {bootmgr} path \EFI\ubuntu\grubx64.efi delete grub boot

bcdedit /set {bootmgr} path \EFI\Microsoft\Boot\bootmgfw.efi set windows boot