CI: Make Sphinx test matrix slot more solid

By installing the designated Sphinx version _after_ the package,
we are absolute sure the right version will be tested.

Author: amotl

.github/workflows/main.yml

@@ -30,6 +30,7 @@ jobs:
 
     env:
       PYTHON: ${{ matrix.python-version }}
+      SPHINX: ${{ matrix.sphinx-version }}
 
     name: >
       Sphinx ${{ matrix.sphinx-version }}
@@ -54,12 +55,12 @@ jobs:
         # https://github.com/pypa/setuptools/blob/main/CHANGES.rst#v6400
         pip install "setuptools>=64" --upgrade
 
-        # Install Sphinx beforehand, to implement test matrix.
-        pip install sphinx==${{ matrix.sphinx-version }}
-
         # Install package in editable mode.
         pip install --use-pep517 --prefer-binary --editable=.[develop,docs,test]
 
+        # Install specific Sphinx version, according to test matrix slot.
+        pip install sphinx==${{ matrix.sphinx-version }}
+
     - name: Run linter and software tests
       run: |
         poe check
@@ -71,6 +72,6 @@ jobs:
       with:
         files: ./coverage.xml
         flags: unittests
-        env_vars: OS,PYTHON
+        env_vars: PYTHON,SPHINX
         name: codecov-umbrella
         fail_ci_if_error: true