Fix packs.download action so it works if default repo branch is not master.
Fix packs.download action so it works if default repo branch is not
master.

tonybaloney/st2,nzlosh/st2,StackStorm/st2,StackStorm/st2,Plexxi/st2,StackStorm/st2,tonybaloney/st2,tonybaloney/st2,StackStorm/st2,nzlosh/st2,nzlosh/st2,nzlosh/st2,Plexxi/st2,Plexxi/st2,Plexxi/st2