fix the doctests installation cache (#66)

* hard-code the python version instead of using a non-existent matrix

* reformatting

Author: keewis

.github/workflows/ci-additional.yml

@@ -24,9 +24,9 @@ jobs:
       uses: actions/cache@v2
       with:
         path: ~/.cache/pip
-        key: ${{ runner.os }}-pip-py${{ matrix.python-version }}-${{hashFiles('ci/requirements/**.txt')}}
+        key: ${{ runner.os }}-pip-py3.8-${{ hashFiles('ci/requirements/**.txt') }}
         restore-keys: |
-          ${{ runner.os }}-pip-py${{ matrix.python-version }}-
+          ${{ runner.os }}-pip-py3.8-
     - name: upgrade pip
       run: |
         python -m pip install --upgrade pip