Switch from script variables to context variables

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v4
         with:
-          python-version: "$PYTHON_VERSION"
+          python-version: ${{env.PYTHON_VERSION}}
 
       - name: Set up Python virtual environment
         run: |
@@ -98,7 +98,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v4
         with:
-          python-version: "$PYTHON_VERSION"
+          python-version: ${{env.PYTHON_VERSION}}
 
       - name: Install system dependencies
         run: |
@@ -172,7 +172,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v4
         with:
-          python-version: "$PYTHON_VERSION"
+          python-version: ${{env.PYTHON_VERSION}}
 
       - name: Append git revision to project version
         run: |