`opam init` for Windows shells #2889

Job	Run time
Analyse	5s
Cygwin	6s
Build-macOS (4.14.1)	44s
Hygiene	26s
Build-Linux (4.08.1)	22s
Test-macOS (4.14.1)	1m 35s
Build-Linux (4.09.1)	34s
Build-Linux (4.10.2)	39s
Build-Linux (4.11.2)	32s
Build-Linux (4.12.1)	26s
Build-Linux (4.13.1)	32s
Build-Linux (5.0.0)	39s
Build-Linux (5.1.1)	26s
Build-Linux (4.14.1)	25s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.1)	1m 36s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	1m 15s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	1m 13s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.1)	2m 6s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.1)	2m 8s
Solvers-Linux	0s
Upgrade-Linux	0s
Test-Linux	0s
Cold-Linux	0s
Solvers-macOS	0s
Upgrade-macOS	0s
	15m 49s

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