[workflows/ci.yml] Change ubuntu-latest to ubuntu-18.04

ubuntu-latest alias will be switched to ubuntu-20.04 soon (see [1]) which lacks python 3.3 and 3.4 (see [2]).

1. https://github.blog/changelog/2020-10-29-github-actions-ubuntu-latest-workflows-will-use-ubuntu-20-04/
2. https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        os: [ubuntu-latest]
+        os: [ubuntu-18.04]
         # TODO: python 2.6
         python-version: [2.7, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, pypy-2.7, pypy-3.6, pypy-3.7]
         python-impl: [cpython]
@@ -26,11 +26,11 @@ jobs:
           ytdl-test-set: download
           run-tests-ext: bat
         # jython
-        - os: ubuntu-latest
+        - os: ubuntu-18.04
           python-impl: jython
           ytdl-test-set: core
           run-tests-ext: sh
-        - os: ubuntu-latest
+        - os: ubuntu-18.04
           python-impl: jython
           ytdl-test-set: download
           run-tests-ext: sh