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fix: make Windows and Linux update downloads cancellable #272

Job	Run time
Racket core (contract tests)	1m 59s
Contract preflight (i18n + release metadata)	4s
macOS host (x64 build)	9m 16s
macOS host (arm64 build)	3m 12s
Linux host (GTK4 build)	4m 45s
Windows host (WinRT build)	4m 58s
	24m 14s