loader_deposit: Adapt default_filename to archive.tar

This is a requirement for the synchronized release of:

• swh.loader.core: 0.19 (deposit loader)
• swh.deposit: 0.10

(on hold as i'm currently fighting with the deposit debian build in buster)

Related to T3070

Test Plan

bin/octocatalog-diff

bin/octocatalog-diff --octocatalog-diff-args --no-truncate-details --to staging worker0.internal.staging.swh.network
Found host worker0.internal.staging.swh.network
WARN     -> Environment "wip-pg-hba-rules-in-yaml" contained non-word characters, correcting name to wip_pg_hba_rules_in_yaml
Cloning into '/tmp/swh-ocd.BlNMzywJ/environments/production/data/private'...
done.
Cloning into '/tmp/swh-ocd.BlNMzywJ/environments/staging/data/private'...
done.
*** Running octocatalog-diff on host worker0.internal.staging.swh.network
I, [2021-02-25T17:06:24.372523 #18563]  INFO -- : Catalogs compiled for worker0.internal.staging.swh.network
I, [2021-02-25T17:06:25.542432 #18563]  INFO -- : Diffs computed for worker0.internal.staging.swh.network
diff origin/production/worker0.internal.staging.swh.network current/worker0.internal.staging.swh.network
*******************************************
  File[/etc/softwareheritage/loader_deposit.yml] =>
   parameters =>
     content =>
      @@ -25,3 +25,4 @@
           username: deposit_basic_auth_swhworker_username
           password: deposit_basic_auth_swhworker_password
      +default_filename: archive.tar
      _
*******************************************
*** End octocatalog-diff on worker0.internal.staging.swh.network

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Migrated from D5151 (view on Phabricator)