[test-upstream] Update flox repo URL (#6780)

Author: dcherian

ci/install-upstream-wheels.sh

@@ -44,6 +44,6 @@ python -m pip install \
     git+https://github.com/pydata/sparse \
     git+https://github.com/intake/filesystem_spec \
     git+https://github.com/SciTools/nc-time-axis \
-    git+https://github.com/dcherian/flox \
+    git+https://github.com/xarray-contrib/flox \
     git+https://github.com/h5netcdf/h5netcdf
 python -m pip install pytest-timeout