opam env: fix carriage return on Cygwin shell #2405

Job	Run time
Analyse	7s
Test-macOS (4.14.1)	10m 36s
Cygwin	4s
Build-Linux (4.08.1)	26s
Build-macOS (4.14.1)	57s
Build-Linux (4.09.1)	22s
Hygiene	50s
Build-Linux (4.10.2)	25s
Build-Linux (4.11.2)	25s
Build-Linux (4.12.1)	25s
Build-Linux (4.13.1)	26s
Build-Linux (5.0.0)	31s
Build-Linux (5.1.0)	25s
Build-Linux (4.14.1)	25s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.1)	1m 27s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	1m 8s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	1m 8s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.1)	1m 56s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.1)	1m 38s
Cold-Linux	0s
Test-Linux	0s
Solvers-Linux	0s
Upgrade-Linux	0s
Solvers-macOS	0s
Upgrade-macOS	0s
	23m 41s

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