housekeeping: pin ubuntu to 20.04 for python 3.6

Ubuntu:latest doesn't include py36 anymore. Keep this until we add 3.11 and drop 3.6.
See:
actions/setup-python#544
rwth-i6/returnn#1226

Author: erikwrede

.github/workflows/tests.yml

@@ -30,7 +30,7 @@ jobs:
           - {name: '3.9', python: '3.9', os: ubuntu-latest, tox: py39}
           - {name: '3.8', python: '3.8', os: ubuntu-latest, tox: py38}
           - {name: '3.7', python: '3.7', os: ubuntu-latest, tox: py37}
-          - {name: '3.6', python: '3.6', os: ubuntu-latest, tox: py36}
+          - {name: '3.6', python: '3.6', os: ubuntu-20.04, tox: py36}
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-python@v4