CI: fix macos unbreaking python #15390

Job	Run time
Arch Linux	17m 8s
CUDA (on Arch)	6m 32s
Fedora	20m 1s
Arch / PyPy	20m 30s
Gentoo	22m 25s
OpenSUSE	15m 32s
Ubuntu Bionic	15m 29s
Ubuntu Rolling (gcc, g++)	16m 20s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	11m 37s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	11m 39s
Ubuntu Rolling (clang, clang++)	19m 19s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	13m 12s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	11m 36s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	11m 33s
	3h 32m 53s

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