CI: update job output mechanism

GH workflow changed job outputs from `echo ::set-output`
to appending to `$GITHUB_OUTPUT` file.

tox-gh-matrix 0.2.0 supports new mechanism

.github/workflows/test.yml

@@ -21,7 +21,7 @@ jobs:
         uses: actions/checkout@v3
       - name: Install tox-gh-matrix
         run: |
-          python -m pip install 'tox<4' 'tox-gh-matrix<0.2'
+          python -m pip install 'tox<4' 'tox-gh-matrix<0.3'
           python -m tox --version
       - name: Generate tox envlist
         id: generate-envlist