Improve the performance of opam update/init on Windows #2850

Job	Run time
Analyse	42s
Build-macOS (4.14.1)	2m 13s
Hygiene	19s
Cygwin	7s
Build-Linux (4.08.1)	40s
Test-macOS (4.14.1)	2m 7s
Build-Linux (4.09.1)	37s
Build-Linux (4.10.2)	37s
Build-Linux (4.11.2)	37s
Build-Linux (4.12.1)	36s
Build-Linux (4.13.1)	41s
Build-Linux (5.0.0)	43s
Build-Linux (5.1.1)	55s
Build-Linux (4.14.1)	44s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.1)	2m 42s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	1m 47s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.1)	1m 35s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.1)	2m 10s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.1)	2m 41s
Upgrade-Linux	0s
Cold-Linux	0s
Test-Linux	0s
Solvers-Linux	0s
Upgrade-macOS	0s
Solvers-macOS	0s
	22m 33s

Provide feedback