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/hydra.sql

@@ -68,6 +68,7 @@ create table Jobsets (
     checkInterval integer not null default 300, -- minimum time in seconds between polls (0 = disable polling)
     schedulingShares integer not null default 100,
     fetchErrorMsg text,
+    forceEval     boolean,
     check (schedulingShares > 0),
     primary key   (project, name),
     foreign key   (project) references Projects(name) on delete cascade on update cascade