staging: Fix object storage configuration for indexers

Related to T2816

Test Plan

octocatalog diff

worker0.staging

iff origin/production/worker0.internal.staging.swh.network current/worker0.internal.staging.swh.network
*******************************************
  File[/etc/softwareheritage/indexer_origin_intrinsic_metadata.yml] =>
   parameters =>
     content =>
      @@ -8,8 +8,10 @@
         url: http://storage1.internal.staging.swh.network:5007/
       objstorage:
      -  cls: pathslicing
      -  args:
      -    root: "/srv/softwareheritage/objects"
      -    slicing: 0:1/1:5
      +  cls: filtered
      +  storage_conf:
      +    cls: remote
      +    url: http://storage1.internal.staging.swh.network:5003/
      +  filters_conf:
      +  - type: readonly
       storage:
         cls: remote
*******************************************
*** End octocatalog-diff on worker0.internal.staging.swh.network

---

Migrated from D4625 (view on Phabricator)