env2mfile: Fix a typo #16651

Job	Run time
Arch Linux	19m 39s
CUDA (on Arch)	7m 49s
Fedora	20m 25s
Gentoo	21m 57s
OpenSUSE	16m 44s
Ubuntu Bionic	15m 19s
Arch / PyPy	20m 10s
Ubuntu Rolling (gcc, g++)	19m 50s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	13m 49s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	15m 1s
Ubuntu Rolling (clang, clang++)	22m 20s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	16m 4s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	13m 50s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 31s
	3h 57m 28s

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