fix coverage_fuzz_corpora to use the target-coverage dir

It seems that cargo fuzz corpus doesn't respect CARGO_TARGET_DIR? but it does respect --target-dir, so use that instead.

Also removed the "use CARGO_TARGET_DIR if set" logic, since it wouldn't have worked in the first place("target-coverage" is hard coded in the line 29 cp command), and you probably still want to use a different target dir even if you have one set.

Also changed the message by running this by itself to be a bit more helpful (so when i want to run this in 6 months time i won't need to look up my old messages on exactly how to do it :) )