Create an index on the hash component of the store path

For derivation_source_files.

sqitch/deploy/derivation_source_files_store_path_hash_index.sql

@@ -0,0 +1,8 @@
+-- Deploy guix-data-service:derivation_source_files_store_path_hash_index to pg
+
+BEGIN;
+
+CREATE INDEX derivation_source_files_hash
+  ON derivation_source_files (substring(store_path from 12 for 32));
+
+COMMIT;