add `-nographic` in `flatcar_production_qemu.sh` for `amd64-usr+x86_64`

[thebigbone]

hello, I noticed that if running the flatcar qemu script on x86_64, it looks for some display manager for starting qemu which fails while running on headless with following error:

gtk initialization failed

I noticed that every other arch has -nographic flag and after adding that, the image boots successfully.

[tormath1]

Hi and thanks for the issue,

I don't think it's an issue to add this -nographic option by default. I regularly use it. This should go there: https://github.com/flatcar/scripts/blob/24d8122b960fcb247697f63236d39849c1957b5a/build_library/qemu_template.sh#L231 and it will be more consistent with the other arches implementation.