set server console to serial, enable guest agent #3

Job	Run time
generate_matrix	1m 18s
update_flake	47s
check_flake	3m 27s
build_images (x86_64, linux, server, proxmox)	1m 45s
build_images (x86_64, linux, server, raw-efi)	1m 10s
build_images (x86_64, linux, server, iso)	1m 42s
build_images (x86_64, linux, bri, proxmox)	3m 19s
build_images (x86_64, linux, bri, raw-efi)	3m 19s
build_images (x86_64, linux, bri, iso)	2m 38s
build_flake (cb)	2m 6s
build_flake (pyEnv)	2m 15s
build_flake (sysdo)	1m 38s
push_update	1s
	25m 25s

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