night ci: add packages for ubuntu 20.04

[pacoxu]

fixes #5240

[k8s-ci-robot]

Hi @pacoxu. Thanks for your PR.

I'm waiting for a containerd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[pacoxu]

Another two places using latest ubuntu are like below:

/Users/xupaco/git/gopath/src/k8s.io/containerd/.github/workflows/codeql.yml:
   19:     runs-on: ubuntu-latest

/Users/xupaco/git/gopath/src/k8s.io/containerd/.github/workflows/nightly.yml:
   12:     runs-on: ubuntu-latest

[pacoxu]

https://github.com/pacoxu/containerd/blob/master/.github/workflows/ci.yml#L40-L68

Test the ci in ci.yaml in my repo: https://github.com/pacoxu/containerd/runs/2164779752?check_suite_focus=true

[aojea]

LGTM

[AkihiroSuda]

use ubuntu-18.04 as the latest now is 20.04

Could you fix the commit message (git commit -a -s --amend)? LGTM then.

[pacoxu]

@AkihiroSuda Done

[AkihiroSuda]

Seems not: ac4a5b5

[pacoxu]

Fixed this time.
😳