Replace the deprecated`set-output` command in the "Scheduled UW IdP Test Matrix" action.

[goulter]

In the "Scheduled UW IdP Test Matrix" action, there are warnings about...
"The set-output command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/"

To do: replace the set-output command with whatever is suggested / not deprecated.