Sync shared fuzzing corpus before exiting the fuzzer

Right now the corpus is only synced to [output/corpus every 10 minutes](https://github.com/srlabs/ziggy/blob/2d49d26032d238c324a4b83a8bb4646d20b9e8fb/src/bin/cargo-ziggy/fuzz.rs#L304). If you stop the fuzzer in between, newer queue entries are still saved in the respective fuzzing engines directories but do not make it into the shared corpus directory. We should sync them before exiting