debian: fix grub package arch selectors for riscv64 #820

Job	Run time
build-cloud-installer-initramfs-job	6m 29s
download-cidata-job	10s
build-cloud-installer-job	14m 49s
test-iso-multidisk-qemu-disk2-with-data-job	1h 49m 57s
test-iso-cf-container-qemu-job	1h 45m 55s
test-iso-multidisk-qemu-disk5-with-data-job	1h 56m 44s
test-iso-multidisk-qemu-empty-disk-job	1h 48m 24s
test-iso-qemu-job	1h 49m 31s
test-iso-cf-qemu-job	1h 38m 42s
	11h 10m 41s

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