TST: install cython nightly using a wheel

.github/workflows/linux.yml

@@ -309,7 +309,7 @@ jobs:
     # TODO: remove cython nightly install when cython does a release
     - name: Install nightly Cython
       run: |
-        pip install git+https://github.com/cython/cython
+        pip install -i https://pypi.anaconda.org/scientific-python-nightly-wheels/simple cython
     # Set PYTHON_GIL=0 to force GIL off during tests and then manually verify
     # importing numpy does not enable the GIL. When f2py grows the ability to
     # declare GIL-disabled support we can just run the tests without the