Add CLI option to specify additional packages for internal Cygwin. #2764

Job	Run time
Analyse	5s
Cygwin	8s
Build-macOS (4.14.1)	3m 8s
Hygiene	16s
Build-Linux (4.08.1)	24s
Test-macOS (4.14.1)	5m 10s
Build-Linux (4.09.1)	30s
Build-Linux (4.10.2)	23s
Build-Linux (4.11.2)	40s
Build-Linux (4.12.1)	33s
Build-Linux (4.13.1)	27s
Build-Linux (5.0.0)	28s
Build-Linux (5.1.1)	23s
Build-Linux (4.14.1)	26s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.1)	1m 35s
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 23s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.1)	1m 55s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.1)	1m 54s
Solvers-Linux	0s
Cold-Linux	0s
Upgrade-Linux	0s
Test-Linux	0s
Upgrade-macOS	0s
Solvers-macOS	0s
	21m 3s

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