Retain older pre-builts

[danobi]

It's unclear how often we'll be changing the kconfigs once we hit steady state.

In the event we change configs somewhat frequently, we may want to retain older pre-builts in the dummy release. That would be so older checkouts of bpftrace repo can still pull the old kernels from here.

In a world where we build kernels inside the flake, it would be a non-issue. But the concern is cache misses and long kernel build times.

Perhaps we can meet somewhere in the middle and version pre-builts here by a checksum of the kconfigs?