Avoid packaging _trial_temp directory (#4326)

Make sure we don't put the _trial_temp directory in the package target
directory.

Fixes https://github.com/matrix-org/synapse/issues/4322

changelog.d/4326.bugfix

+Avoid packaging _trial_temp directory in -py3 debian packages
+

debian/build_virtualenv

@@ -42,8 +42,7 @@ tmpdir=`mktemp -d`
 trap "rm -r $tmpdir" EXIT
 
 cp -r tests "$tmpdir"
-cd debian/matrix-synapse-py3
 
 PYTHONPATH="$tmpdir" \
-    ./opt/venvs/matrix-synapse/bin/python \
+    debian/matrix-synapse-py3/opt/venvs/matrix-synapse/bin/python \
         -B -m twisted.trial --reporter=text -j2 tests