CI matrix change: add Windows.Server.Core.20H2 - Second attempt

[aik-jahoda]

The original CI matrix change was reverted (#60040) because of a docker issue:

Unable to pull image mcr.microsoft.com/dotnet-buildtools/prereqs:windowsservercore-20H2-helix-amd64-20210922171304-edc3733
Exit Code:-4

This is attempt to investigate and re-revert it back.

https://dev.azure.com/dnceng/public/_build/results?buildId=1405926&view=results

part of #59572

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Issue Details

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The original CI matrix change was reverted (#60040) because of a docker issue:

Exit Code:-4  ```

This is attempt to investigate and re-revert it back.

<table>
  <tr>
    <th align="left">Author:</th>
    <td>aik-jahoda</td>
  </tr>
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    <th align="left">Assignees:</th>
    <td>-</td>
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  <tr>
    <th align="left">Labels:</th>
    <td>

`area-Infrastructure-libraries`

</td>
  </tr>
  <tr>
    <th align="left">Milestone:</th>
    <td>-</td>
  </tr>
</table>
</details>

[aik-jahoda]

@krwq , I try fix the docker issue. We all all green, can we trigger rolling builds on this PR?

[krwq]

you will need to push your branch to dotnet, i.e.:
git push origin cimatrix20h2:cimatrix20h2 (what's before : is your local branch name, what's after : is remote branch name you want to have or already have) - make sure to not add space before or after : as that might delete the branch
then go to https://dev.azure.com/dnceng/public/_build?definitionId=686&_a=summary and click Run pipeline and choose your branch name and click Run - please share the link for your build in the PR description once it's started

[aik-jahoda]

Latest commit is passing in runtime CI: febb494

[danmoseley]

if it works..

[ericstj]

This PR introduced regressions #62205 and is being reverted in #62253