System unavailable: ftp.gnu.org

- Please put the system name in the title of this issue.

- Link to any log file showing the problem: https://ci.adoptium.net/job/build_image_updater/91/consoleFull or https://ci.adoptium.net/job/VagrantPlaybookCheck/OS=CentOS7,label=vagrant/2141/console
```
#11 606.3 fatal: [localhost]: FAILED! => {"changed": false, "dest": "/tmp/make-4.1.tar.gz", "elapsed": 10, "msg": "Request failed: ", "url": "https://ftp.gnu.org/gnu/make/make-4.1.tar.gz"}
#11 606.3
```

- Please describe the issue:
First noticed during some work I was doing a couple of days ago ([Slack reference](https://adoptium.slack.com/archives/C53GHCXL4/p1754989960770679)) but I didn't raise an issue at the time as I expected it to get quickly resolved. Unfortunately it is still causing issues today.

In my local testing I have switched `https://ftp.gnu.org/gnu` to `https://www.mirrorservice.org/sites/ftp.gnu.org/gnu` - If this persists I may put in a PR to switch that temporarily.

Start with the linked build_image_updater and VagrantPlaybookCheck logs, then search the repository for the ftp.gnu.org/gnu URL and the make-4.1 download. Confirm whether the upstream outage persists and identify the affected build path; done means the relevant CI checks can download the dependency reliably again.

Written by the indexing model from the issue text.