include post timestamp in ikiwiki GUIDs

The recent changes page can include multiple posts with the same GUID,
as the GUID is simply the URL of the page. In blog posts, that's
usually fine because we don't want to act every time a blog post
is *modified* but for things like link checks or archival, it's
actually important to fetch pages on each update.

This will require the user to run `--catchup` on the feeds with that
plugin enabled after upgrade.

We simply prefix the GUID with a ISO-8601 formatted timestamp so that
changes (above seconds increments) are considered
parent 08a79fc6
Pipeline #33469218 failed with stage
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