try the recommended syntax for pip installing deps

Author: keewis

ci/requirements/environment.yaml

@@ -12,4 +12,7 @@ dependencies:
   - numpy
   - pip
   - pip:
-      - -i https://pypi.anaconda.org/scientific-python-nightly-wheels/simple xarray
+      - --pre
+      - -i https://pypi.anaconda.org/scientific-python-nightly-wheels/simple
+      - --extra-index-url https://pypi.org/simple
+      - xarray