Fix a panic related to batch GC
Fix a panic related to batch GC

`deleteJobVersions` does concurrent modifications to iterated items
while iterating, by deleting job versions while it's iterating on them,

burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,nak3/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,nak3/nomad,nak3/nomad,nak3/nomad,hashicorp/nomad,dvusboy/nomad,nak3/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,nak3/nomad,dvusboy/nomad,hashicorp/nomad,burdandrei/nomad,dvusboy/nomad,nak3/nomad