nixos-ami: update nvme_core.io_timeout for linux kernel >= 4.15
nixos-ami: update nvme_core.io_timeout for linux kernel >= 4.15

NixOS 20.03 is built on kernel 5.4 and 19.09 is on 4.19, so we should update
this option to the highest value possible, per linked upstream instructions from
Amazon.

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