I resolved the issue by restarting the depot builder.
You can do this in fly.io/dashboard, select the organization on which the stuck builder resides (top left selector), then on the left-side menu there’s an “app builders” option.
Click on the “reset” button corresponding to the sick builder, that’ll destroy it and a new one will be provisioned on your next deploy.
It’s unbelievable how Fly can ship something like this. The Depot builder is failing at least once a day for a couple of hours. No idea how Fly can ship this as a “default”/ production ready. Yesterday I couldn’t deploy a hotfix for 6 hours, now the same is happening again. Please do something about this Fly! @joshua-fly