fix external_project failing due to the drive letter in the path on Windows #17365

Job	Run time
Arch / PyPy	21m 19s
Arch Linux	20m 17s
Ubuntu Rolling (gcc, g++)	19m 42s
CUDA (on Arch)	7m 52s
Fedora	18m 28s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 59s
Gentoo	23m 22s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	14m 22s
OpenSUSE	16m 36s
Ubuntu Rolling (clang, clang++)	22m 28s
Ubuntu Bionic	15m 15s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	15m 38s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 26s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	13m 53s
	3h 56m 37s

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