Support testing build determinism

Builds can now specify the attribute "isDeterministic = true" to tell
Hydra to build with build-repeat > 0. If there is a mismatch between
rounds, the step / build fails with a suitable status.

Maybe this should be a meta attribute, but that makes it invisible to
hydra-queue-runner, and it seems reasonable to make a claim of
mandatory determinism part of the derivation (since e.g. enabling this
flag should trigger a rebuild).

src/hydra-queue-runner/queue-monitor.cc

@@ -418,6 +418,7 @@ Step::ptr State::createStep(ref<Store> destStore,
     step->drv = readDerivation(drvPath);
 
     step->preferLocalBuild = step->drv.willBuildLocally();
+    step->isDeterministic = get(step->drv.env, "isDetermistic", "0") == "1";
 
     step->systemType = step->drv.platform;
     {