Switch back to upstream dh-virtualenv (#7621)

Upstream have merged our changes
(https://github.com/spotify/dh-virtualenv/pull/300), so let's switch back to it
instead of using our fork.

changelog.d/7621.misc

+Switch to upstream `dh-virtualenv` rather than our fork for Debian package builds.

docker/Dockerfile-dhvirtualenv

@@ -28,7 +28,7 @@ RUN env DEBIAN_FRONTEND=noninteractive apt-get install \
 
 # fetch and unpack the package
 RUN mkdir /dh-virtualenv
-RUN wget -q -O /dh-virtualenv.tar.gz https://github.com/matrix-org/dh-virtualenv/archive/matrixorg-20200519.tar.gz
+RUN wget -q -O /dh-virtualenv.tar.gz https://github.com/spotify/dh-virtualenv/archive/ac6e1b1.tar.gz
 RUN tar -xv --strip-components=1 -C /dh-virtualenv -f /dh-virtualenv.tar.gz
 
 # install its build deps. We do another apt-cache-update here, because we might