Fix type issues with the location ids

On one code path, they were handled as numbers, whereas elsewhere they were
handled as strings. This led to the package-metadata code trying to insert
duplicate entries.

Instead, just handle them as strings everywhere.

guix-data-service/model/package-metadata.scm

@@ -92,7 +92,7 @@ WHERE packages.id IN (
                            (value->quoted-string-or-null synopsis) ","
                            (value->quoted-string-or-null description) ","
                            (value->quoted-string-or-null home_page) ","
-                           (number->string location_id)
+                           location_id
                            ")")))
                        metadata-rows)
                   ",")