nmbug-status: Don't require write access

nmbug-status: Don't require write access

The database in only used for notmuch.Query, so there's no need for
write access.  This allows nmbug-status to run while the database is
being updated, without raising:

  A Xapian exception occurred opening database: Unable to get write lock on …: already locked
  Traceback (most recent call last):
    File "./nmbug-status", line 182, in <module>
      db = notmuch.Database(mode=notmuch.Database.MODE.READ_WRITE)
    File "/…/notmuch/database.py", line 154, in __init__
      self.open(path, mode)
    File "/…/notmuch/database.py", line 214, in open
      raise NotmuchError(status)
  notmuch.errors.XapianError