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GHA: bump cygwin/cygwin-install-action to 6.1 #148

Job	Run time
Windows	36s
Cygwin	1m 52s
macOS	14s
Linux (default, valgrind, test-memory)	3m 0s
Linux (clang sanitizers, clang, test, clang, -fsanitize=address,undefined,signed-integer-overflow...	32s
	6m 14s

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