Constrain the permissions in `fetch-testfiles` #696

Job	Run time
Fetch Test Files	12s
Android / build (Debug)	53s
Android / build (Release)	44s
Android / build (RelWithDebInfo)	46s
iOS / build (Debug)	2m 24s
iOS / build (Release)	2m 37s
iOS / build (RelWithDebInfo)	2m 2s
tvOS / build (Debug)	2m 11s
tvOS / build (Release)	2m 37s
tvOS / build (RelWithDebInfo)	57s
macOS (arm64) / build (Debug)	3m 30s
Linux (x86_64) / build (Debug)	3m 48s
Linux (arm64) / build (Debug)	4m 11s
macOS (arm64) / build (Release)	1m 45s
Linux (x86_64) / build (Release)	2m 50s
macOS (arm64) / build (RelWithDebInfo)	3m 49s
Linux (arm64) / build (Release)	3m 17s
Linux (x86_64) / build (RelWithDebInfo)	3m 19s
Linux (arm64) / build (RelWithDebInfo)	3m 1s
macOS (x86_64) / build (Debug)	5m 2s
macOS (x86_64) / build (Release)	3m 17s
macOS (x86_64) / build (RelWithDebInfo)	4m 11s
Windows (x86_64) / build (Debug)	9m 50s
Windows (x86_64) / build (Release)	8m 31s
Windows (x86_64) / build (RelWithDebInfo)	9m 38s
	1h 25m 22s

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