Allow deleting distfiles if distfiles is a symlink

[ryandesign]

When I originally prevented deletions if distfiles was a symlink in f151333 I was considering replacing the distfiles directory on each worker with a symlink to the master distfiles volume mounted over AFP/SMB to save disk space on the workers. I didn't end up trying that, but on the Apple Silicon workers I have now made distfiles a symlink to a shared directory where I do want deletions to occur, so I've changed the condition so now deletions can occur unless ports.tar.gz exists; that would only exist on the master. I added a slash to the end of the distfiles path since otherwise find won't descend into the symlinks.