We can use gst_uri_from_string_with_base () to join base url
and the fragment url path.
The previous method of forming base url in update_base_url(),
by looking for the string 'manifest' or 'Manifest' is insufficient.
A query may include these string in their paths and thus an invalid
base url string will be kept.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8193>
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