Update installer to explicitly use ffmpeg 6.1

[gabek]

The existing linux static binaries we had been using didn't have ffmpeg 6.1 to pull from. And another place didn't have a stable location that we would be able to pull 6.1 from as time goes on.

So I pulled the current 6.1 and copied them to our own storage so we'd be able to rely on those urls.

This PR should not be merged until https://github.com/owncast/owncast/pull/3476 is released.

[stale[bot]]

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