Make sure download.test does not fail due to a checksum collision in the download cache #4048

Job	Run time
Analyse	6s
Hygiene	21s
Cygwin	7s
Build-Linux (4.08.1)	2m 27s
Build-macOS (4.14.2)	4m 11s
Build-Linux (4.09.1)	3m 11s
Build-macOS (5.3.0)	3m 20s
Test-macOS (4.14.2)	28m 22s
Build-Linux (4.10.2)	2m 27s
Test-macOS (5.3.0)	23m 39s
Build-Linux (4.11.2)	3m 20s
Build-Linux (4.12.1)	3m 39s
Build-Linux (4.13.1)	3m 13s
Build-Linux (5.0.0)	3m 8s
Build-Linux (5.1.1)	2m 18s
Build-Linux (5.2.1)	2m 24s
Build-Linux (4.14.2)	2m 29s
Build-Linux (5.3.0)	2m 34s
Build-Windows (x86_64-pc-cygwin, x86_64-pc-cygwin, 4.14.2)	36m 7s
Build-Windows (i686-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	41m 43s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 4.14.2)	42m 49s
Build-Windows (i686-pc-windows, x86_64-pc-cygwin, 4.14.2)	45m 45s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 4.14.2)	45m 32s
Build-Windows (x86_64-w64-mingw32, x86_64-pc-cygwin, 5.3.0)	40m 16s
Build-Windows (x86_64-pc-windows, x86_64-pc-cygwin, 5.3.0)	42m 42s
Cold-Linux	5m 20s
Solvers-Linux (z3, 4.14.2)	1m 13s
Test-Linux (4.14.2)	11m 14s
Doc-Linux (4.14.2)	1m 37s
Solvers-Linux (z3, 5.3.0)	1m 6s
Upgrade-Linux (4.14.2)	1m 23s
Solvers-Linux (0install, 4.14.2)	1m 36s
Test-Linux (5.3.0)	9m 51s
Doc-Linux (5.3.0)	1m 13s
Upgrade-Linux (5.3.0)	2m 15s
Solvers-Linux (0install, 5.3.0)	1m 39s
Upgrade-macOS (4.14.2)	1m 46s
Solvers-macOS (z3, 4.14.2)	1m 25s
Upgrade-macOS (5.3.0)	1m 57s
Solvers-macOS (z3, 5.3.0)	1m 26s
Solvers-macOS (0install, 4.14.2)	1m 34s
Solvers-macOS (0install, 5.3.0)	1m 40s
	7h 14m 25s

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