CI: Fix helix test results reporting

[radical]

PR #73060 broke uploading of helix test results. This was caused by the
change:

      <HelixPostCommands>@(HelixPostCommand)</HelixPostCommands>

This is overwriting the existing value of $(HelixPostCommands), which
gets set to have the upload script invocation in https://github.com/dotnet/arcade/blob/34dff939b4a91e4693f78a856e0e055c1a3f3fba/src/Microsoft.DotNet.Helix/Sdk/tools/azure-pipelines/AzurePipelines.MonoQueue.targets#L8-L15 at evaluation time.

Fix by appending to the property.

Thanks to ChadNedzlek for finding the cause!

Fixes #74699 .

[ghost]

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Issue Details

---

PR #73060 broke uploading of helix test results. This was caused by the
change:

      <HelixPostCommands>@(HelixPostCommand)</HelixPostCommands>

This is overwriting the existing value of $(HelixPostCommands), which
gets set to have the upload script invocation in https://github.com/dotnet/arcade/blob/34dff939b4a91e4693f78a856e0e055c1a3f3fba/src/Microsoft.DotNet.Helix/Sdk/tools/azure-pipelines/AzurePipelines.MonoQueue.targets#L8-L15 at evaluation time.

Fix by appending to the property.

Thanks to ChadNedzlek for finding the cause!

Fixes #74699 .

Author:	radical
Assignees:	-
Labels:	blocking, area-Infrastructure
Milestone:	-

[ChadNedzlek]

Make sure the test count in the PR build comes back up to 3 millionish, and this should be good.

[akoeplinger]

Looks like we had a new failing test sneak in while this was broken, disabled it (see #74781)

[radical]

Does this need to wait for these jobs that we are re-running?

[steveisok]

Does this need to wait for these jobs that we are re-running?

I think it's ok to merge because it's high impact.