Generate workload msis in parallel

[mmitche]

The workload generation step generates each msi serially, rather than in parallel. This creates a pretty decent sized long pole in the runtime build (~45 mins) and should be resolved.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[joeloff]

We can close this. We confirmed that build time for workloads decreased ~50% from 44 minutes to 20 minutes