ci/gha: bump ubuntu to 20.04
ci/gha: bump ubuntu to 20.04

Otherwise GHA complains:

> Ubuntu-latest workflows will use Ubuntu-20.04 soon. For more details,
> see https://github.com/actions/virtual-environments/issues/1816

Apparently, despite the wording, it is already using 20.04, but it
is nice to silence the warning regardless.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

moby/sys