`wp rewrite structure` fails to properly set tag / category rewrite rules

Steps to reproduce:

```
wp rewrite structure /blog/%year%/%monthnum%/%day%/%postname%/ --category-base=section --tag-base=topic
wp rewrite flush
wp rewrite structure /blog/%year%/%monthnum%/%day%/%postname%/ --category-base=sections --tag-base=topics
wp rewrite list
```

Expected: the rewrite rules include plural versions of regex for category / tag base.

Actual: the rewrite rules still use the singular versions of the regex.

Had to include an extra `wp rewrite flush` in the functional tests to get them to pass: https://github.com/wp-cli/wp-cli/commit/610e0e20ac62fc28bf4c1bad5be1788078d247f0#diff-39480501507816ca5cf3aee44a64518fR8