Don't write a cover sibling per download (--dont-save-cover)

calibredb export writes <uuid>.jpg next to <uuid>.<format> by
default, leaving an unused cover image in /tmp/calibre_web for
every download that goes through do_calibre_export. The cover
file is never read by calibre-web (cover serving uses a separate
path), so it is pure waste of disk space and IO.

Pass --dont-save-cover, paired with the existing --dont-write-opf,
to skip the unwanted side-effect at the source. Verified that
none of the three do_calibre_export call sites (download via
do_download_file, email via tasks/mail.py, kepubify via
tasks/convert.py) read the cover sibling.

cps/embed_helper.py

@@ -36,7 +36,8 @@ def do_calibre_export(book_id, book_format):
         if config.config_calibre_split:
             my_env['CALIBRE_OVERRIDE_DATABASE_PATH'] = os.path.join(config.config_calibre_dir, "metadata.db")
         library_path = config.get_book_path()
-        opf_command = [calibredb_binarypath, 'export', '--dont-write-opf', '--with-library', library_path,
+        opf_command = [calibredb_binarypath, 'export', '--dont-write-opf', '--dont-save-cover',
+                       '--with-library', library_path,
                        '--to-dir', tmp_dir, '--formats', book_format, "--template", "{}".format(temp_file_name),
                        str(book_id)]
         p = process_open(opf_command, quotes, my_env)