Normalize nixexpr{input,path} from builds to jobsetevals.

Duplicating this data on every record of the builds table cost
approximately 4G of duplication.

Note that the database migration included took about 4h45m on an
untuned server which uses very slow rotational disks in a RAID5 setup,
with not a lot of RAM. I imagine in production it might take an hour
or two, but not 4. If this should become a chunked migration, I can do
that.

Note: Because of the question about chunked migrations, I have NOT
YET tested this migration thoroughly enough for merge.

src/sql/hydra.sql

@@ -162,13 +162,6 @@ create table Builds (
     isChannel     integer not null default 0, -- meta.isHydraChannel
     isCurrent     integer default 0,
 
-    -- Copy of the nixExprInput/nixExprPath fields of the jobset that
-    -- instantiated this build.  Needed if we want to reproduce this
-    -- build.  FIXME: this should be stored in JobsetEvals, storing it
-    -- here is denormal.
-    nixExprInput  text,
-    nixExprPath   text,
-
     -- Priority within a jobset, set via meta.schedulingPriority.
     priority      integer not null default 0,
 
@@ -466,6 +459,8 @@ create table JobsetEvals (
     nrSucceeded   integer, -- set lazily when all builds are finished
 
     flake         text, -- immutable flake reference
+    nixExprInput  text, -- name of the jobsetInput containing the Nix or Guix expression
+    nixExprPath   text, -- relative path of the Nix or Guix expression
 
     foreign key   (project) references Projects(name) on delete cascade on update cascade,
     foreign key   (project, jobset) references Jobsets(project, name) on delete cascade on update cascade