I can think of a workaround, but
subversion provides no easy local way to determine the proxy, for which we haven’t had the handler take care of the setup as with other components.
Something like this might work:
echo "http-proxy-host=my.proxy" > $HOME/.subversion
echo "http-proxy-port=80" > $HOME/.subversion
# Maybe also http-proxy-username=[username]
# And #http-proxy-password=[password]
For which, it might be something buildd could just eventually end up setting up regardless of environment.