Reuse object files from previous CI runs

[albinahlback]

Could potentially reduce the load on the runners quite a bit.

However, we would need to have the following in mind:

1. Have different caches for each runner.
2. Use new cache whenever compiler tools/OS/whatever are changed between runs.
3. Reuse flint.h if no changes in flint.h.in was made.

Perhaps other things has to be taken into account as well.