swh-loader-svn: Build rsvndump dependency

This will allow the svn loader to be adapted to use that new dependency tool instead of svnrdump.

$ docker build -t swh-loader-svn .
$ docker run -ti --rm --user root --entrypoint rsvndump swh-loader-svn --version
rsvndump 0.6.1
Copyright (C) 2008-present Jonas Gehring <jonas@jgehring.net>
Released under the GNU General Public License.

This is inlined with the actual svn dockerfile. At some point, this will probably get outsourced in its own Dockerfile (so as to be reused by other images, swh-environment:/docker for example).

cc @teams/sysadmin @anlambert

Related to https://forge.softwareheritage.org/D433