update NixOS fdisk instructions based on my flow (18.03)

[KiaraGrouwstra]

The existing installation instructions did not quite match what I ran into on a clean install (NixOS 18.03) -- this PR alters the instructions to reflect my experience.

[grahamc]

I'm not sure why these are an improvement. My understanding is your clean install was overtop an existing set of partitions, and not actually to a clean disk. Am I misunderstanding?

[KiaraGrouwstra]

Hi, thanks for asking.

It's not that I wanted to preserve partitions -- I was wiping/repartitioning my SSD as part of this install.
I can't tell what was the reason the original instructions gave me issues -- they somehow did, then I documented my steps for myself / others with similar issues (?).
I can't tell which of these only occurred for me and which could be more general. If not useful, feel free to close.

Changes included:

• partition table type MBR -> GPT: helped me solve an error File system "/boot" is not on a GPT partition table. edit: also error warning: this GPT partition label contains no BIOS Boot Partition; embedding won't be possible
• fdisk -> gdisk -- iirc this was needed for the partition table difference above.
• add button presses for some missing prompts, though potentially tweaked toward gdisk.
• boot partition size: helped me solve error NixOS fails to install #34508.
• vfat signature warnings: ignore if applicable.

[afrepues]

Upon start, gdisk will attempt to convert any existing from MBR or BSD disklabels into a GPT. Then using the o function you can clean out that GPT to start anew.

[KiaraGrouwstra]

I'll close as it doesn't appear clear whether this would be of use.