enumerating release jobset names is single-threaded and slow

[ghost]

The only known way to enumerate the names of all packages/jobs in a release is the one used by ofborg for the eval-checks:

• https://github.com/nixos/ofborg#running-meta-checks-locally

This is obscenely slow and requires an absurd amount of memory. It is entirely single-threaded with zero parallelism. It peaks at 60gbytes of memory and takes 41 minutes on a 3ghz cpu.

This accounts for the vast majority of the cycle time for our CI.