Fix opam on OpenBSD #680

Job	Run time
Analyse	6s
Build-macOS (4.14.1)	2m 40s
Cygwin	5s
Build-Linux (4.08.1)	3m 35s
Hygiene	24s
Test-macOS (4.14.1)	24m 12s
Build-Linux (4.09.1)	3m 45s
Build-Linux (4.10.2)	3m 27s
Build-Linux (4.11.2)	3m 36s
Build-Linux (4.12.1)	1m 51s
Build-Linux (4.13.1)	44s
Build-Linux (5.0.0)	38s
Build-Linux (5.1.1)	36s
Build-Linux (4.14.1)	37s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.1)	26m 11s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	35m 26s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	35m 42s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.1)	38m 5s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.1)	40m 39s
Upgrade-Linux	0s
Cold-Linux	0s
Solvers-Linux	0s
Test-Linux	0s
Upgrade-macOS (4.14.1)	1m 46s
Solvers-macOS (z3, 4.14.1)	1m 22s
Solvers-macOS (0install, 4.14.1)	1m 18s
	3h 46m 45s

Provide feedback