add some debug logging for skip-existing #1666

Job	Run time
test (ubuntu-latest)	1m 32s
Build x86_64-unknown-linux-musl	7m 49s
test (macos-latest)	1m 54s
Build aarch64-unknown-linux-musl	8m 10s
test (windows-latest)	4m 13s
Build powerpc64le-unknown-linux-gnu	8m 14s
Build x86_64-apple-darwin	9m 53s
Build aarch64-apple-darwin	10m 9s
Build x86_64-pc-windows-gnu	16m 39s
Build x86_64-pc-windows-msvc	15m 40s
Post-process	2s
	1h 24m 15s

Provide feedback