CI: Avoid docker containers in favor of prebuilt binaries

[FreezyLemon]

Reasoning: See jrottenberg/ffmpeg#410 and the PR linked there. TL;DR: An update to the docker images (which was done from a PR branch, not documented and can't be found anywhere on main) broke our CI & the one response I have gotten has not been reassuring.

Since the problem is effectively a showstopper for this repo, let's switch to something that shouldn't break so easily, even if it's a bit more work to set up initially.

Changes:

• Replace jrottenberg/ffmpeg docker container with prebuilt binaries from https://github.com/wang-bin/avbuild
  • This needs a bit of patching due to an issue in the pkgconfig files
  • otherwise, just extract to some directory and then set PKG_CONFIG_PATH for compile time and LD_LIBRARY_PATH for runtime
• Remove 5.0 and 6.0 from CI as they are EOL upstream
• Adjust a few tests that relied on the external libraries included in FFmpeg

[FreezyLemon]

Thankfully, this didn't uncover any bugs in the previous PRs :D