github-actions: bump docker/setup-buildx-action from 3.8.0 to 3.9.0 (… #237

Job	Run time
packet-verification-low-power	8m 57s
thread-1-4-gcc-m32	11m 50s
thread-1-4-gcc-m64	11m 38s
thread-1-4-clang-m32	10m 37s
channel-manager-csl	3m 4s
thread-1-4-clang-m64	9m 52s
expects	29m 14s
packet-verification-1-1-on-1-4	10m 50s
thread-1-4-posix	9m 38s
upload-coverage	1s
	1h 45m 41s

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