rebuilder: more clever rebuild if output is not in cache

[JulienMalka]

We should either find a way not to use nix-build --check whose semantics are dodgy, or wrap it inside a logic that does different thing if output is not in cache

[raboof]

by 'dodgy semantics' do you mean the fact that it'll only call the diff hook for the first failing result (#34), or something else?

Indeed it might be interesting to do something different here (or improve the semantics upstream ;) )

[JulienMalka]

By dodgy semantics, I mean that there is no way in Nix to simply rebuild locally a derivation, without asking Nix to check the results. This creates problem when the derivation is not in a cache for example, and makes it also not really idempotent in that case.

In our instance, I think we should just check if the derivation the rebuilder wants to build is in the cache or not / present locally or not and adapt our behavior.