[ci] fix conda env creation in 'regular' CI job (fixes #6282) #6817

Job	Run time
Python - oldest supported versions (ubuntu-latest)	3m 28s
regular (macOS-latest, Python 3.9)	15m 44s
sdist (macOS-latest, Python 3.10)	19m 16s
bdist (macOS-latest, Python 3.7)	23m 36s
if-else (macOS-latest, Python 3.9)	4m 48s
mpi source (macOS-latest, Python 3.10)	20m 44s
mpi pip (macOS-latest, Python 3.11)	18m 39s
mpi wheel (macOS-latest, Python 3.8)	23m 10s
all-python-package-jobs-successful	2s
	2h 9m 27s

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