lastmod Field Always Set to Current Date

[doxsch]

Description:

I've encountered a bug in the next-sitemap package when using it with next export. The field is always set to the current date instead of the date when the content actually changed. This could lead to Google ignoring this information in the future.

Steps to Reproduce:

1. Install and configure next-sitemap with next export.
2. Generate the sitemap
3. Check the field in the generated sitemap.

Expected Behavior:

The field should be set to the actual modification date of the content.

Actual Behavior:

The field is always set to the current date.