It looks like `callCabalProjectToNix` can fail in its `cabal` call with the following error: ``` open: permission denied (Permission denied) ``` Then running the same derivation locally works. If the error is deferred there is no easy way on hydra to retry. This change makes it so only solvers errors (which should hopefully be deterministic) are deferred. All other errors will go back to being eval time errors meaning they should be automatically retried at the next hydra eval.