ytdl_hook: propogate ytdl.path out so that mp.get_opt can see it

[bitingsock]

Allows other scripts to use ytdl.path without having to discover it themselves.

[github-actions]

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Windows

• mpv-i686-w64-mingw32
• mpv-x86_64-w64-mingw32

[sfan5]

1. This is only set when ytdl_hook ran at least once, how useful is that for other scripts?
2. IMO user-data is the better place to store this

[bitingsock]

Yes, I remembered only afterwards that the hook does not run unless needed. Likely not as useful as I had hoped.

[kasper93]

Could be useful for scripts like https://github.com/christoph-heinrich/mpv-quality-menu where you have to set ytdl_ver separately. I don't know what is the correct solution, but I guess there is little bit of value.