Clean the sysroot destination directory before copying the content into it #1254

Job	Run time
build (libgccjit.so, master)	9m 1s
build (libgccjit12.so, gcc12, --no-default-features, TEST_FLAGS='-Cpanic=abort -Zpanic-abort-test...	1m 50s
build (libgccjit_without_int128.so, master-without-128bit-integers)	9m 40s
success_failures	2s
	20m 33s

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