Specify DB_CHARSET as default-character-set for wp-db-import

A DB export followed by a DB import of the same file was resulting in corrupted UTF-8 characters. The script was being run in the varying-vagrant-vagrants environment which has `latin1` as the default charset, which was not the same as the `DB_CHARSET` of `utf-8`:

```
mysql> show variables like "character_set_%";
+--------------------------+----------------------------+
| Variable_name            | Value                      |
+--------------------------+----------------------------+
| character_set_client     | latin1                     |
| character_set_connection | latin1                     |
| character_set_database   | latin1                     |
| character_set_filesystem | binary                     |
| character_set_results    | latin1                     |
| character_set_server     | latin1                     |
| character_set_system     | utf8                       |
| character_sets_dir       | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
```