Switch to busybox mirror to make the CI less flaky #684

Job	Run time
ci (torvalds, x86_64, defconfig+debug, llvm, src)	15m 33s
ci (torvalds, x86_64, defconfig, llvm, src)	15m 36s
ci (torvalds, x86_64, defconfig, gcc, src)	30m 40s
ci (torvalds, x86_64, defconfig+debug, gcc, src)	30m 38s
	1h 32m 27s

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