Build the wheel, install it on a separate machine, run the tests. Profit.

[NicolasHug]

This PR adds the following jobs to our OSS CI:

• Build a wheel (these are the same wheel(s) that would be updated on PyPI eventually)
• Save that wheel as an artifact (see actions/upload-artifact@v4)
• Spin up a new machine, download the wheel that was previously built
• Install torchcodec from that wheel: that's exactly what users will eventually do when they pip install torchcodec from PyPI
• Install FFmpeg - this can be done at any point, but we do it here to ensure there's no build/install dependency on FFmpeg
• Run the torchcodec tests to ensure the wheel installation went fine.

If this works, it means we're 90% done with the packaging on x86 Linux.

There is some overlap with the already-existing "Unit Test / python" job. I think we can get rid of those, but I'll leave that for a follow-up PR.

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@NicolasHug merged this pull request in 1bc888d.