Cannot obtain build Ids inside Powershell Task

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I set the option Store the triggered build id's in a variable.
However in a subsequent powershell task I'm not able to obtain the value of the variable in any way.

If I list the variables using:

Get-ChildItem Env:

No variable like the expected documented ones appear in the list.

How can I obtain the launched build Ids inside a subsequent powershell task?