Install development version of dask in downstream tests (#472)

Author: jrbourbeau

.github/workflows/testing.yml

@@ -99,7 +99,7 @@ jobs:
     if: "contains(github.event.pull_request.labels.*.name, 'ci distributed') || contains(github.event.pull_request.labels.*.name, 'ci downstream')"
     env:
       PROJECT: distributed
-      TEST_REQUIREMENTS: cryptography pytest pytest-asyncio<0.14.0 pytest-timeout pytest-rerunfailures numpy pandas mock bokeh fsspec>=0.3.3 aiohttp pyarrow
+      TEST_REQUIREMENTS: cryptography pytest pytest-asyncio<0.14.0 pytest-timeout pytest-rerunfailures numpy pandas mock bokeh fsspec>=0.3.3 aiohttp pyarrow git+https://github.com/dask/dask
       PROJECT_URL: https://github.com/dask/distributed.git
     strategy:
       matrix: