Don't clobber built jar when downloading reference jar

Ever since the path returned by get_final_file_name() was converted to an absolute path in !23 (merged), we seem to have been overwriting the built jar with the reference jar during verification. This meant that we were comparing the hash of the reference jar to itself.

This bug came to light because there aren't yet any reference jars for macOS, so the built jar was being clobbered by an empty file.