Ubuntu 26.04 image build and runner updates

[koksay]

Add Ubuntu 26.04 (Resolute) GHA runner images and runners

Extends the OCI VM runner image pipeline to build, test, release and serve Ubuntu 26.04 alongside the existing 24.04 images, and adds a parallel set of cncf-ubuntu-26.04-* runner scale sets.

Image build (build-actions-vm.yml, ci/gha-runner-vm*/main.go)

• build-x86 / build-arm64 are matrixed over ubuntu-version: [24.04, 26.04] (codenames noble / resolute). Each leg checks for its own latest actions/runner-images release, downloads the matching build.ubuntu-<ver>[-arm64].pkr.hcl template and names the image ubuntu-<ver>-<arch>-gha-image.
• x86: the cloud image URL now uses cloud-images.ubuntu.com/releases/<codename>/release/… and passes a matching --isoChecksum (the default noble/current/SHA256SUMS doesn't list the releases/ filenames).
• arm64: --os-version is now passed explicitly (the OCI builder otherwise defaults to 24.04).
• Fixed Apt.Tests.ps1 failure (Apt sources resolve through the mirror list). Upstream's configure-apt-sources.sh only rewrites http://azure.archive.ubuntu.com/ubuntu/ → mirror+file:…; on OCI cloud-init writes <region>-ad-1.clouds.archive.ubuntu.com/ubuntu/ and under qemu it writes archive.ubuntu.com/ubuntu (no trailing slash), so the sed never matched.
• Packer bumped to 1.16.1.
• Build artifacts are suffixed per version: image-ocid-{x86,arm64}-<codename>, object-name-{x86,arm64}-<codename>, image-tag-{x86,arm64}-<codename>.

Test stage

• run-kind-with-arc and test-runner are matrixed over arch × os-version, passing os-version to the run-kind-with-arc action. Scale sets are named oci-ci-test-<run_id>-<os-version>-<arch> and the poller waits on test-runner (<arch>, <os-version>), so 24.04 and 26.04 legs don't collide.

Release / copy / import

• release-*-image, copy-*-image-to-regions and import-*-image-on-regions are matrixed over version (× region where applicable), consume the per-codename artifacts and release under ubuntu-<ver>-<arch>-gha-image in us-sanjose-1, us-ashburn-1, us-phoenix-1 and us-chicago-1.

Runners (ci/cluster/**/vm-runners)

• Existing 24.04 manifests (incl. cncf-ubuntu-bm-arm and the GPU runners) now pass --os-version=24.04 explicitly instead of relying on the cloudrunner default.

• New cncf-ubuntu-26.04-<cpu>-<mem>-<arch> scale sets for every existing size, in the same clusters as their 24.04 counterparts:

  • ci/cluster/oci/vm-runners: 2-8, 16-64 (x86 + arm) and 4-16, 8-32, 24-96, 32-128 (arm)
  • oke-gha-chi: 4-16, 32-128 (x86)
  • oke-gha-phx: 8-32, 24-96 (x86)

  They use the label cncf-ubuntu-resolute-<size>-<arch> as an alias and don't carry the legacy oracle-vm-* labels.

• ci/README.md lists the new runs-on labels.

Rollout notes

• The 26.04 scale sets resolve the released ubuntu-26.04-<arch>-gha-image, so they'll only start serving jobs after the first successful 26.04 run of build-actions-vm.yml has released and imported the image into all regions.
• Known follow-up: the image_exists gate is still a single any-version signal. On the daily schedule, when one version is already current and the other needs a rebuild, the current version's test legs will find no rc- image. Per-version gating (dynamic matrix from a plan job) will be handled separately.

[github-actions]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: koksay

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[koksay]

/hold
/uncc @jeefy @riaankleinhans

[koksay]

/uncc @mrbobbytables @riaankleinhans
/cc @jeefy

Still in progress, I will ping when the pr is ready

[koksay]

The images are successfully built: https://github.com/cncf/automation/actions/runs/36815874306

The PR is ready for review, @jeefy

/hold cancel

[jeefy]

/lgtm

Great job @koksay