Fix compatibility with fmtlib 8
LCG 103 features fmtlib 8.1.1 which is more picky about what it is asked to format.
As this is needed for LCG 103 and that I'm going to test LCG 103 in lhcb-head-2 (lhcb-core/LHCbNightlyConf!982 (merged)), I'm adding the label.