fix BUILD_IMAGE unbound var #37

Job	Run time
update_flake	51s
generate_matrix	1m 0s
build_flake (cb)	1m 41s
build_images (x86_64, linux, server, proxmox)	16m 4s
check_flake	3m 56s
build_flake (pyEnv)	1m 38s
build_images (x86_64, linux, server, raw-efi)	15m 47s
build_flake (sysdo)	1m 36s
build_images (x86_64, linux, server, iso)	39m 1s
build_images (x86_64, linux, bri, proxmox)	4m 48s
build_images (x86_64, linux, bri, raw-efi)	4m 34s
build_images (x86_64, linux, bri, iso)	37m 50s
push_update	0s
	2h 8m 46s

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