Add ARM64 overcloud host image builds

[cityofships]

An alternative to #1497.

• Don't give a choice to operators for picking a platform - images targeting both platforms are always built and uploaded, so they stay close to each other.
• Make distro and platform builds parallel so the workflow finishes faster.
• Does not use cross-arch at all. Leafcloud builds will not be supported, since there's no arm64 hypervisors there.

Depends on: stackhpc/stackhpc-release-train#441

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.

[cityofships]

https://github.com/stackhpc/stackhpc-kayobe-config/actions/runs/18563982840

[Alex-Welsh]

It would be nice to include some validation to make sure we're actually building images for the right arch.

Unfortunately it doesn't come out of qemu-img info, the cleanest I have of finding it is:

sudo apt install -y libguestfs-tools
sudo guestfish --ro -a path/to/image -i file /bin/bash | grep x86-64

or

sudo apt install -y libguestfs-tools
sudo guestfish --ro -a path/to/image -i file /bin/bash | grep aarch64

[Alex-Welsh]

@cityofships looks like you have a conflict. Needs a rebase, and probably worth squashing while you're at it