Index the edge after the column that carries it exists

On a store with the old shape the table is not re-created, so an index declared
beside it is built on a column the migration has not added yet.

modules/mesh-catalog/store.ts

@@ -93,8 +93,6 @@ CREATE TABLE IF NOT EXISTS built_against (
   against_reference  text NOT NULL,
   PRIMARY KEY (module, commit_sha, against_reference)
 );
-
-CREATE INDEX IF NOT EXISTS built_against_target ON built_against (against_reference);
 `;
 
 // The graph first keyed its edges on a module and a commit, which the builder does not know and
@@ -107,6 +105,11 @@ DELETE FROM built_against WHERE against_reference IS NULL;
 ALTER TABLE built_against DROP COLUMN IF EXISTS against_module;
 ALTER TABLE built_against DROP COLUMN IF EXISTS against_commit;
 ALTER TABLE built_against ALTER COLUMN against_reference SET NOT NULL;
+
+-- After the column exists, and not in the schema above: on a store that still has the old shape,
+-- the table is not created, so an index named in the same breath would be built on a column that
+-- is not there yet.
+CREATE INDEX IF NOT EXISTS built_against_target ON built_against (against_reference);
 `;
 
 export class Graph {