Switch to busybox mirror to make the CI less flaky #683

Job	Run time
ci (torvalds, x86_64, defconfig, gcc, src)	29m 20s
ci (torvalds, x86_64, defconfig, llvm, src)	16m 14s
ci (torvalds, x86_64, defconfig+debug, gcc, src)	31m 26s
ci (torvalds, x86_64, defconfig+debug, llvm, src)	15m 1s
	1h 32m 1s

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