e2e: add mid-rollout image change test

[ptalgulk01]

Summary

• Add TestMidRolloutImageChange e2e test that verifies changing the target image mid-rollout does not cause unnecessary reboots
• Build a second update image (node:update2) for testing distinct image transitions
• Add NodeImageUpdate2DigestedPullSpec() and NodeImageUpdate2Digest() helpers to the e2e test environment

The test provisions two worker nodes, starts a rollout to one image, the switches the target to a different image while one node is rebooting. It verifies both nodes converge to the final image and uses journalctl --list-boots to confirm the non-rebooting node did not wastefully reboot into the first image.

Partial progress on #69

Test plan

• make e2e V=1 RUN=TestMidRolloutImageChange passes (183s)
• Boot count verified: non-rebooting node rebooted exactly once
• make -n build-update-image and make -n e2e dry-runs confirm
  Makefile correctness
• All three images (latest, update, update2) produce distinct
  digests