ci: Use --find-links to install jax and jaxlib on Python 3.7 (#2422)

* As Python 3.7 versions of jax and jaxlib are no longer hosted on PyPI, use
  the --find-links option and the Google archive of jax release wheels to
  install the dependencies.

.github/workflows/ci.yml

@@ -40,7 +40,14 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
 
+    - name: Install dependencies (Python 3.7)
+      if: matrix.python-version == '3.7'
+      run: |
+        python -m pip install --upgrade pip setuptools wheel
+        python -m pip install --find-links https://storage.googleapis.com/jax-releases/jax_releases.html --upgrade .[test]
+
     - name: Install dependencies
+      if: matrix.python-version != '3.7'
       run: |
         python -m pip install --upgrade pip setuptools wheel
         python -m pip install --upgrade .[test]