ytdl_hook.lua: set metadata with use_manifests=yes

With use_manifests=yes EDL is not used so metadata never gets set
through the tags mechanism. Thankfully the metadata property was
recently made writable so use that instead.

The existing metadata variable is inlined to fix linting.

Fixes https://github.com/mpv-player/mpv/pull/17761#issuecomment-4306070651