[ci] Migrate webpack tests back to arm runners (#95549)
I partially rolled back CI jobs because we ran out of runners. IT
increased the concurrency from 50 to 100, so I've been slowly migrating
back to arm runners while watching that we don't end up with long queue
times again.
Thread: https://vercel.slack.com/archives/C0B0LKK5QD7/p1782838752523649
Average job queue time in the last day has been 59 seconds:
https://github.com/vercel/next.js/actions/metrics/performance?dateRangeType=DATE_RANGE_TYPE_CUSTOM&range=1783296000000-1783296000000
