When manually scheduling an eval, force re-instantiation of store derivations

Without this, if (failed or aborted) derivations have been
garbage-collected, there is no way to restart them, which is very
annoying. Now we set a forceEval flag in the jobset to cause it to be
re-evaluated even if none of the inputs have changed.

src/sql/upgrade-51.sql

@@ -0,0 +1 @@
+alter table Jobsets add column forceEval boolean;