GHA/distcheck: disable `man-db/auto-update`
Make sure to not rebuild man pages after purging system curl, to make
the job faster and avoid timeouts:
```
Sun, 14 Sep 2025 10:16:28 GMT Removing curl (8.5.0-2ubuntu10.6) ...
Sun, 14 Sep 2025 10:16:28 GMT Processing triggers for man-db (2.12.0-4build2) ...
Sun, 14 Sep 2025 10:21:22 GMT (Reading database ... 218629 files and directories currently installed.)
```
Ref: https://github.com/curl/curl/actions/runs/
17709785947/job/
50326910814?pr=18535#step:3:19
Closes #18544