Add skip-changelog PR label for bump VM PRs (#8627)

This commit adds the `skip-changelog` GitHub label for PRs created by
automation for Bump VM image PRs. Without it, the mandatory
[fragment-in-pr](https://github.com/elastic/elastic-agent/blob/main/.github/workflows/fragment-in-pr.yml)
check is failing thus preventing automerge.

Author: dliappis

.ci/updatecli/updatecli-bump-vm-images.yml

@@ -26,6 +26,7 @@ actions:
       labels:
         - dependencies
         - backport-skip
+        - skip-changelog
       title: '[{{ requiredEnv "BRANCH_NAME" }}][Automation] Bump VM Image version to {{ source "latestVersion" }}'
 
 sources: