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/state.hh

@@ -35,6 +35,7 @@ typedef enum {
     bsUnsupported = 9,
     bsLogLimitExceeded = 10,
     bsNarSizeLimitExceeded = 11,
+    bsNotDeterministic = 12,
     bsBusy = 100, // not stored
 } BuildStatus;
 
@@ -139,6 +140,7 @@ struct Step
     nix::Derivation drv;
     std::set<std::string> requiredSystemFeatures;
     bool preferLocalBuild;
+    bool isDeterministic;
     std::string systemType; // concatenation of drv.platform and requiredSystemFeatures
 
     struct State