linux

fix external_project failing due to the drive letter in the path on Windows #17032

Job	Run time
Arch / PyPy	19m 31s
Ubuntu Rolling (gcc, g++)	20m 30s
Arch Linux	20m 7s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 37s
CUDA (on Arch)	7m 6s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 9s
Fedora	17m 46s
Ubuntu Rolling (clang, clang++)	22m 31s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	16m 8s
Gentoo	22m 11s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	14m 24s
OpenSUSE	16m 45s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 36s
Ubuntu Bionic	14m 56s
	3h 53m 17s

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