Explicitly declare hierarchical=false for post_tag in create_initial_taxonomies().

In certain edge cases, such as during the automated test suite,
create_initial_taxonomies() can be called in a situation where a non-default
permastruct setting has been saved outside of wp-admin. This causes the
'hierarchical' setting for the post_tag taxonomy to remain unset, leading to
PHP notices in get_term_link().

See #29827.
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