Support Nix depexts with `opam env` #4146

Job	Run time
Analyse	7s
Cygwin	5s
Build-Linux (4.08.1)	2m 21s
Hygiene	18s
Build-Linux (4.09.1)	2m 22s
Build-macOS (4.14.2)	2m 49s
Test-macOS (4.14.2)	13m 22s
Build-Linux (4.10.2)	2m 18s
Build-macOS (5.3.0)	2m 51s
Test-macOS (5.3.0)	17m 20s
Build-Linux (4.11.2)	2m 17s
Build-Linux (4.12.1)	3m 21s
Build-Linux (4.13.1)	3m 36s
Build-Linux (5.0.0)	2m 35s
Build-Linux (5.1.1)	2m 21s
Build-Linux (5.2.1)	2m 30s
Build-Linux (4.14.2)	2m 18s
Build-Linux (5.3.0)	2m 34s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.2)	34m 54s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	39m 24s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	41m 8s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.2)	44m 27s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.2)	45m 57s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 5.3.0)	43m 12s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 5.3.0)	41m 16s
Cold-Linux	5m 14s
Test-Linux (4.14.2)	7m 48s
Doc-Linux (4.14.2)	1m 10s
Solvers-Linux (z3, 4.14.2)	1m 1s
Test-Linux (5.3.0)	7m 23s
Doc-Linux (5.3.0)	1m 11s
Solvers-Linux (z3, 5.3.0)	2m 7s
Upgrade-Linux (4.14.2)	1m 15s
Solvers-Linux (0install, 4.14.2)	2m 59s
Upgrade-Linux (5.3.0)	1m 18s
Solvers-Linux (0install, 5.3.0)	1m 38s
Upgrade-macOS (4.14.2)	1m 45s
Solvers-macOS (z3, 4.14.2)	1m 18s
Upgrade-macOS (5.3.0)	1m 48s
Solvers-macOS (z3, 5.3.0)	1m 6s
Solvers-macOS (0install, 4.14.2)	1m 19s
Solvers-macOS (0install, 5.3.0)	1m 21s
	6h 37m 24s

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