cmake: could not find TBB in ATLAS builds

With the current Gaudi master, we are seeing the following cmake configuration error in ATLAS:

-- Setting up LCG release using: /cvmfs/sft.cern.ch/lcg/releases/LCG_100_ATLAS_1/LCG_generators_x86_64-centos7-gcc8-dbg.txt
-- Found TBB: /cvmfs/sft.cern.ch/lcg/releases/LCG_100_ATLAS_1/tbb/2020_U2/x86_64-centos7-gcc8-dbg (found version "2020.2") 
[...]
-- Found ROOT: /cvmfs/sft.cern.ch/lcg/releases/LCG_100_ATLAS_1/ROOT/v6.24.00/x86_64-centos7-gcc8-dbg/cmake (found version 6.24.00)
-- Could NOT find TBB (missing: TBB_DIR)
[...]
-- The following REQUIRED packages have not been found:

 * TBB (required version >= 2019.0.11007.2)

This started appearing after !1209 (merged) got merged. My first suspicion was this is related to LCG_100, but it already happened on LCG_98.

cc @clemenci @akraszna