Code review comment for ~paride/jenkins-launchpad-plugin:py3

The root of the problem seems to be:

[ 267.086787] cloud-init[1461]: Processing triggers for linux-image-4.4.0-170-generic (4.4.0-170.199) ...
[ 267.410038] cloud-init[1461]: /etc/kernel/postinst.d/zz-flash-kernel:
[ 267.411989] cloud-init[1461]: flash-kernel: deferring update (trigger activated)
[ 267.423883] cloud-init[1461]: Processing triggers for flash-kernel (3.0~rc.4ubuntu62.2) ...
[ 267.563889] cloud-init[1461]: Can't find /boot/vmlinuz-4.4.0-170-generic or /boot/initrd.img-4.4.0-170-generic
[ 267.579645] cloud-init[1461]: dpkg: error processing package flash-kernel (--configure):
[ 267.591371] cloud-init[1461]: subprocess installed post-installation script returned error exit status 1
[ 267.596234] cloud-init[1461]: Errors were encountered while processing:
[ 267.599077] cloud-init[1461]: flash-kernel

Likely to be caused by this commit:

https://git.launchpad.net/curtin/commit/?id=04adfd9a83964ccec944b0f015143ea3b5b21473

Possible workaround: restrict the "run update-initramfs once" logic to arm64 until we find what goes wrong on arm64 and s390x.