fix(ci): Wait for preview to load to avoid 423 errors when deleting file

[come-nc]

See nextcloud/server#42306 (comment)

[come-nc]

/backport to stable28

[backportbot]

The backport to stable28 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable28
git pull origin stable28

# Create the new backport branch
git checkout -b backport/2806/stable28

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 54b34784

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/2806/stable28

Error: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.