Use a separate db connection in channel-derivations-by-system-promise

As this can run in parallel with something else using conn.

guix-data-service/jobs/load-new-guix-revision.scm

@@ -2214,11 +2214,14 @@ SELECT 1 FROM derivation_source_file_nars WHERE derivation_source_file_id = $1"
     (define channel-derivations-by-system-promise
       (fibers-delay
        (lambda ()
-         (channel->source-and-derivations-by-system
-          conn
-          channel-for-commit
-          fetch-with-authentication?
-          #:parallelism parallelism))))
+         (with-postgresql-connection
+          "load-new-guix-revision channel->source-and-derivations-by-system"
+          (lambda (channel-conn)
+            (channel->source-and-derivations-by-system
+             channel-conn
+             channel-for-commit
+             fetch-with-authentication?
+             #:parallelism parallelism))))))
 
     (define guix-revision-id-promise
       (fibers-delay