The clean option doesn't clean the makepkg BUILDDIR

Steps to reproduce: 1. Set `clean = true` in `~/.config/aura/config.toml` 2. Set `BUILDDIR=/tmp/makepkg` in `/etc/makepkg.conf` 3. Build a package. 4. The directory at `/tmp/makepkg/<pkg_name>` will contain a file. The fix is simple enough. Pass the `-c` option to makepkg when clean is set in the configuration file.