use gnu parallel to trigger parallel checkupdates pipelines
use gnu parallel to trigger parallel checkupdates pipelines

Batch checkupdate invocations into calls with 100 apps. This should
allow more than enough headroom for gitlab CI runner limits.
Currently it'll spawn 34 child pipelines, these each complete in about
15-20 min.

f-droid/fdroiddata,f-droid/fdroiddata