[CI] Run cython-latest on stock ubuntu-20.04

.github/workflows/main.yml

@@ -279,17 +279,12 @@ jobs:
 
   cython-latest:
     name: Test pre-release version of Cython
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
       name: Checkout the repository
       with:
         submodules: recursive
-    - name: Setup python
-      uses: actions/setup-python@v2
-      with:
-        python-version: '3.8'
-        architecture: x64
     - name: Install Apt dependencies
       run: |
         sudo apt update