ci: use X_PYTHON_VERSION for package job

This avoids having to update the CI python version in multiple places.

.github/workflows/ci.yml

@@ -94,10 +94,10 @@ jobs:
 
  steps:
  - uses: actions/checkout@v2
- - name: Set up Python 3.10
+ - name: Set up Python ${{ env.X_PYTHON_VERSION }}
  uses: actions/setup-python@v2
  with:
- python-version: "3.10"
+ python-version: ${{ env.X_PYTHON_VERSION }}
  - name: Install dependencies
  run: |
  python -m pip install --upgrade pip