Fix SourceLinkUrl calculation in another place

[MattGal]

Fallout from #7850 again. I will actually wait until this clears validation to port to 6.0 .

To double check:

• The right tests are in and and the right validation has happened. Guidance: https://github.com/dotnet/core-eng/tree/master/Documentation/Validation Since this can't be easily tested as it stands (needs a repo from devdiv.vs): I rigged up the following demo that uses the msbuild content as shown in the PR.

<Project>
  <ItemGroup>
    <SourceRoot Include="/__w/1/s/.packages/" /> <!-- don't fail on empty -->
    <SourceRoot Include="/__w/1/s/.packages2/">  <!-- correctly lowercase and de-trusted a devdiv  url, AND hit it with update-->
      <ScmRepositoryUrl>https://devdiv.visualstudio.com/MattGal/arcade-validation-trusted.git</ScmRepositoryUrl>
    </SourceRoot>
    <SourceRoot Include="/__w/1/s/.packages3/">  <!-- correctly do nothing to a non devdiv  url -->
      <ScmRepositoryUrl>https://www.github.com/MattGal/arcade-validation-trusted.git</ScmRepositoryUrl>
    </SourceRoot>

  </ItemGroup>
  <Target Name="Build">

    <ItemGroup>
      <SourceRoot> <!-- test the first piece -->
          <ScmRepositoryUrl Condition=" '$([MSBuild]::ValueOrDefault(`%(SourceRoot.ScmRepositoryUrl)`, ``).Contains(`devdiv.visualstudio`))' == 'true' ">$([MSBuild]::ValueOrDefault(`%(SourceRoot.ScmRepositoryUrl)`, ``).ToLower().Replace(`-trusted`,``))</ScmRepositoryUrl>
      </SourceRoot>
    </ItemGroup>

    <ItemGroup>
      <SourceRoot Update="@(SourceRoot)"> <!-- test the second piece -->
        <ScmRepositoryUrl Condition="$([MSBuild]::ValueOrDefault(`%(SourceRoot.ScmRepositoryUrl)`, ``).Contains(`devdiv.visualstudio`))">$([MSBuild]::ValueOrDefault(`%(SourceRoot.ScmRepositoryUrl)`, ``).ToLower().Replace(`mattgal`,`okthisworkedtoo`))</ScmRepositoryUrl>
      </SourceRoot>
    </ItemGroup>

    <Message Importance="High" Text="@(SourceRoot->'%(Identity):%(ScmRepositoryUrl)')" />
  </Target>
</Project>

Output:

Microsoft (R) Build Engine version 16.11.0+0538acc04 for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

  /__w/1/s/.packages/:;/__w/1/s/.packages2/:https://devdiv.visualstudio.com/okthisworkedtoo/arcade-validation.git;/__w/1/s/.packages3/:https://www.github.com/MattGal/arcade-validation-trusted.git