base-buster/Dockerfile: Add missing subversion/mercurial dependencies

Fix loader-svn [2] and loader-mercurial's [1] current ci build failure (master branch).

• [1] https://jenkins.softwareheritage.org/job/DLDHG/job/tests/409/console

• [2] https://jenkins.softwareheritage.org/job/DLDSVN/job/tests/397/console

Reproduced locally by removing respectively those tools and trigger tox in both repos.

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Migrated from D2239 (view on Phabricator)