github: pin ubuntu to 20.04 to make py36 work

Ubuntu versions newer that 20.04 do not support Python 3.6 as per
https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

Change-Id: I92d8e762a7d05e4b0d6d6e90944ceedbbfa74e57
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/389117
Tested-by: Daniel Kutik <daniel.kutik@lavawerk.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Daniel Kutik <daniel.kutik@lavawerk.com>
diff --git a/.github/workflows/test-ci.yml b/.github/workflows/test-ci.yml
index 0b9ef72..bf53b40 100644
--- a/.github/workflows/test-ci.yml
+++ b/.github/workflows/test-ci.yml
@@ -13,7 +13,8 @@
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        # ubuntu-20.04 is the last version that supports python 3.6
+        os: [ubuntu-20.04, macos-latest, windows-latest]
         python-version: ['3.6', '3.7', '3.8', '3.9', '3.10']
     runs-on: ${{ matrix.os }}