Cache flake-based jobset evaluations

src/sql/hydra.sql

@@ -527,6 +527,8 @@ create table JobsetEvals (
     nrBuilds      integer,
     nrSucceeded   integer, -- set lazily when all builds are finished
 
+    flake         text, -- immutable flake reference
+
     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
 );

src/sql/upgrade-57.sql

@@ -4,3 +4,4 @@ alter table Jobsets add column type integer default 0;
 alter table Jobsets add column flake text;
 alter table Jobsets add check ((type = 0) = (nixExprInput is not null and nixExprPath is not null));
 alter table Jobsets add check ((type = 1) = (flake is not null));
+alter table JobsetEvals add column flake text;