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/root/build.tt

@@ -69,6 +69,8 @@ FOR step IN steps; IF step.busy; busy = 1; END; END;
                 <span class="error">Log limit exceeded</span>
               [% ELSIF step.status == 11 %]
                 <span class="error">Output limit exceeded</span>
+              [% ELSIF step.status == 12 %]
+                <span class="error">Non-deterministic build</span>
               [% ELSIF step.errormsg %]
                 <span class="error">Failed: [% HTML.escape(step.errormsg) %]</span>
               [% ELSE %]