switch cron job to 18.04

.github/workflows/ci_cron_weekly.yml

@@ -11,7 +11,7 @@ jobs:
   # Testing links in documents is a good example of something to run on a schedule
   # to catch links that stop working for some reason.
   doc_test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python to build docs with sphinx

.github/workflows/ci_tests.yml

@@ -84,23 +84,3 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       run: |
         tox -e ${{ matrix.toxenv }}
-
-  #
-  # TEMPORARY link test
-  #
-  doc_test:
-    runs-on: ubuntu-18.04
-    # runs-on: macos-latest
-    steps:
-    - uses: actions/checkout@v2
-    - name: Set up Python to build docs with sphinx
-      uses: actions/setup-python@v2
-      with:
-        python-version: 3.8
-    - name: Install base dependencies
-      run: |
-        python -m pip install --upgrade pip
-        python -m pip install tox
-    - name: Check links in docs using tox
-      run: |
-        tox -e linkcheck