HLTUtils: fix unit test for clang build

Frank Winklmeierrequested to merge
fwinkl/athena:hltutils_clang into master

Our clang builds are currently building against the gcc tdaq builds and there is not 'clang' platform tag in OKS. Simply revert to a (random) existing tag to make the SW repository generation work. But this will obviously not be usable in a partition.

Fixes ATR-22849.

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