factor out `with` #78

Job	Run time
generate_matrix	1m 2s
update_flake	57s
check_flake	17m 12s
build_images (x86_64, linux, server, proxmox)	9m 59s
build_images (x86_64, linux, server, raw-efi)	10m 25s
build_images (x86_64, linux, server, iso)	10m 10s
build_images (x86_64, linux, bri, proxmox)	33m 54s
build_images (x86_64, linux, bri, raw-efi)	16m 58s
build_images (x86_64, linux, bri, iso)	16m 57s
build_flake (cb)	1m 28s
build_flake (pyEnv)	1m 32s
build_flake (sysdo)	1m 37s
push_update	0s
	2h 2m 11s

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