deposit.loader: Make archive.tar the default_filename

The main change from using tar in the deposit has been released so we can flip from archive.zip to archive.tar.

Build fails for unrelated reasons (master one fails as well)

The real improvment would be swh-deposit#3094 but this is left as an easy-hack task.

Related to swh-deposit#3094 Related to swh-deposit#3070 (closed)

Depends on !181 (closed)

Test Plan

tox

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Migrated from D5204 (view on Phabricator)