[ci] fix conda env creation in 'regular' CI job (fixes #6282) #6820

Job	Run time
Python - oldest supported versions (ubuntu-latest)	3m 18s
regular (macOS-latest, Python 3.9)	14m 55s
sdist (macOS-latest, Python 3.10)	14m 11s
bdist (macOS-latest, Python 3.7)	18m 51s
if-else (macOS-latest, Python 3.9)	5m 47s
mpi source (macOS-latest, Python 3.10)	14m 44s
mpi pip (macOS-latest, Python 3.11)	12m 27s
mpi wheel (macOS-latest, Python 3.8)	18m 44s
all-python-package-jobs-successful	3s
	1h 43m 0s

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