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/sql/hydra.sql

@@ -211,6 +211,7 @@ create table Builds (
     --   9 = unsupported system type
     --  10 = log limit exceeded
     --  11 = NAR size limit exceeded
+    --  12 = build or step was not deterministic
     buildStatus   integer,
 
     size          bigint,