document options (and fix rsync timestamps?) #73

Job	Run time
update_flake	43s
generate_matrix	1m 1s
check_flake	17m 13s
build_images (x86_64, linux, server, proxmox)	10m 40s
build_images (x86_64, linux, server, raw-efi)	11m 23s
build_images (x86_64, linux, server, iso)	9m 50s
build_images (x86_64, linux, bri, proxmox)	16m 48s
build_images (x86_64, linux, bri, raw-efi)	17m 12s
build_images (x86_64, linux, bri, iso)	15m 50s
build_flake (cb)	57s
build_flake (pyEnv)	1m 31s
build_flake (sysdo)	1m 0s
push_update	0s
	1h 44m 8s

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