bp2::ext::env() Fixes #478

Job	Run time
alpine-linux	21m 27s
windows (msvc-14.0, 14,latest, 32,64, windows-2019)	6h 0m 9s
posix (gcc-5, 11,14,1z, ubuntu-18.04, g++-5)	1s
posix (gcc-6, 11,14,1z, ubuntu-18.04, g++-6)	1s
posix (gcc-7, 11,14,17, ubuntu-18.04)	1s
windows (msvc-14.2, 14,17,20,latest, 32,64, windows-2019)	6h 0m 9s
windows (msvc-14.3, 14,17,20,latest, 32,64, windows-2022)	6h 0m 8s
windows (clang-win, 14,17,latest, 32,64, windows-2022)	6h 0m 9s
windows (gcc, 11,14,17,2a, 64, windows-2019)	1h 3m 8s
posix (gcc-10, 11,14,17,2a, ubuntu-20.04, g++-10)	48m 52s
posix (gcc-12, 11,14,17,20,2b, ubuntu-22.04, g++-12)	1h 12m 4s
posix (clang, clang++-3.9, 11,14, ubuntu-18.04, clang-3.9)	1s
posix (clang, clang++-4.0, 11,14, ubuntu-18.04, clang-4.0)	1s
posix (clang, clang++-5.0, 11,14,1z, ubuntu-18.04, clang-5.0)	1s
posix (clang, clang++-6.0, 11,14,17, ubuntu-18.04, clang-6.0)	1s
posix (clang, clang++-7, 11,14,17, ubuntu-18.04, clang-7)	1s
posix (clang, clang++-8, 11,14,17, ubuntu-20.04, clang-8)	32m 40s
posix (clang, clang++-9, 11,14,17,2a, ubuntu-20.04, clang-9)	46m 17s
posix (clang, clang++-10, 11,14,17,2a, ubuntu-20.04, clang-10)	48m 14s
posix (clang, clang++-11, 11,14,17,2a, ubuntu-20.04, clang-11)	48m 39s
posix (clang, clang++-12, 11,14,17,2a, ubuntu-20.04, clang-12)	47m 33s
posix (clang, clang++-13, 11,14,17,20,2b, ubuntu-22.04, clang-13)	1h 1m 38s
posix (clang, clang++-14, 11,14,17,20,2b, ubuntu-22.04, clang-14)	1h 2m 47s
posix (clang, 11,14,17,2a, macos-11)	1s
	1d 9h 14m 3s

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