ExternalProgram: add cmd_array to complete the offering #16861

Job	Run time
Arch Linux	20m 13s
Arch / PyPy	20m 10s
CUDA (on Arch)	7m 3s
Fedora	19m 36s
Ubuntu Rolling (gcc, g++)	19m 30s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 37s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	13m 35s
Gentoo	22m 28s
Ubuntu Rolling (clang, clang++)	21m 55s
OpenSUSE	16m 4s
Ubuntu Bionic	15m 37s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 17s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 14s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 48s
	3h 53m 7s

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