Install linux-headers/linux-image although not having backports. #277

Job	Run time
download-cidata-job	9s
build-cloud-installer-initramfs-job	7m 11s
build-cloud-installer-job	13m 31s
test-iso-cf-container-qemu-job	1h 41m 19s
test-iso-cf-qemu-job	1h 36m 58s
test-iso-qemu-job	1h 47m 48s
test-iso-multidisk-qemu-disk2-with-data-job	1h 48m 50s
test-iso-multidisk-qemu-empty-disk-job	1h 48m 42s
test-iso-multidisk-qemu-disk5-with-data-job	1h 51m 14s
	10h 55m 42s

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