benchmarks/libcelero: Fix build

Fix breakage with clang10 by switching to clang-8:
error: loop variable 'udm' of type 'const std::1::shared_ptr<celero::UserDefinedMeasurement>' creates a copy from type 'const std::1::shared_ptr<celero::UserDefinedMeasurement>'
https://github.com/DigitalInBlue/Celero/issues/142

Reported by: fallout