Improve chunking when inserting derivation inputs

Chunk the values inserted in the query, rather than the derivations involved,
as this is more consistent.

guix-data-service/model/derivation.scm

@@ -1381,7 +1381,6 @@ WHERE derivation_source_files.store_path = $1"
         #f)))
 
 (define (insert-derivation-inputs conn derivation-ids derivations)
-  (define (process-chunk derivation-ids derivations)
   (let ((query-parts
          (append-map!
           (lambda (derivation-id derivation)
@@ -1406,7 +1405,8 @@ WHERE derivation_source_files.store_path = $1"
           derivation-ids
           derivations)))
 
-      (unless (null? query-parts)
+    (chunk-for-each!
+     (lambda (query-parts-chunk)
        (exec-query
         conn
         (string-append
@@ -1414,18 +1414,15 @@ WHERE derivation_source_files.store_path = $1"
 INSERT INTO derivation_inputs (derivation_id, derivation_output_id)
 SELECT vals.derivation_id, derivation_outputs.id
 FROM (VALUES "
-          (string-join query-parts ", ")
+         (string-join query-parts-chunk ", ")
          ") AS vals (derivation_id, file_name, output_name)
 INNER JOIN derivations
   ON derivations.file_name = vals.file_name
 INNER JOIN derivation_outputs
   ON derivation_outputs.derivation_id = derivations.id
- AND vals.output_name = derivation_outputs.name")))))
-
-  (chunk-for-each! process-chunk
-                   200
-                   (list-copy derivation-ids)
-                   (list-copy derivations)))
+ AND vals.output_name = derivation_outputs.name")))
+     1000
+     query-parts)))
 
 (define (select-from-derivation-source-files store-paths)
   (string-append