Fix borg break-lock ignoring BORG_REPO env var

borg/archiver.py

@@ -1316,7 +1316,7 @@ class Archiver:
                                           formatter_class=argparse.RawDescriptionHelpFormatter,
                                           help='break repository and cache locks')
         subparser.set_defaults(func=self.do_break_lock)
-        subparser.add_argument('location', metavar='REPOSITORY',
+        subparser.add_argument('location', metavar='REPOSITORY', nargs='?', default='',
                                type=location_validator(archive=False),
                                help='repository for which to break the locks')