Fix twisted trunk with sytest

Install twisted trunk into the correct virtual env and use offline mode
so that twisted trunk doesn't get overridden with the pinned version of
twisted.

Signed-off-by: Sean Quah <seanq@element.io>

.github/workflows/twisted_trunk.yml

@@ -59,16 +59,19 @@ jobs:
 
     steps:
       - uses: actions/checkout@v2
-      - uses: matrix-org/setup-python-poetry@dmr/try-extras
-        with:
-          python-version: "3.x"
-          extras: "all"
       - name: Patch dependencies
-        run: .ci/patch_for_twisted_trunk.sh
+        run: >-
+          ln -s -T /venv /src/.venv &&
+          .ci/patch_for_twisted_trunk.sh &&
+          unlink /src/.venv
         working-directory: /src
       - name: Run SyTest
         run: /bootstrap.sh synapse
         working-directory: /src
+        env:
+          # Use offline mode to avoid reinstalling the pinned version of
+          # twisted.
+          OFFLINE: 1
       - name: Summarise results.tap
         if: ${{ always() }}
         run: /sytest/scripts/tap_to_gha.pl /logs/results.tap