external-project: Test env kwarg #16996

Job	Run time
Arch Linux	20m 3s
CUDA (on Arch)	7m 15s
Fedora	17m 54s
Gentoo	21m 47s
OpenSUSE	16m 39s
Ubuntu Bionic	15m 9s
Arch / PyPy	20m 3s
Ubuntu Rolling (gcc, g++)	19m 40s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 54s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 59s
Ubuntu Rolling (clang, clang++)	22m 39s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 24s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 55s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 2s
	3h 52m 23s

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