Boot from another system for example from Qubes OS installer USB disk.
Find out on which disk did you install Qubes OS:
fdisk -l
For example it’s /dev/sda.
Find out which partition on /dev/sda has EFI System type:
fdisk -l /dev/sda
For example it’s /dev/sda1.
Mount EFI partition:
mount /dev/sda1 /mnt
Copy files from /mnt/EFI/qubes/ to /mnt/EFI/BOOT/:
cp -r /mnt/EFI/qubes/* /mnt/EFI/BOOT/.
Change filenames grubx64.efi → BOOTX64.efi and grub.cfg → BOOTX64.cfg:
mv /mnt/EFI/BOOT/grubx64.efi /mnt/EFI/BOOT/BOOTX64.efi
mv /mnt/EFI/BOOT/grub.cfg /mnt/EFI/BOOT/BOOTX64.cfg