rust-ffmpeg-splitter: Disable AV1 encoder for Lambda target

[samohovets]

Summary

• Disable the libaom AV1 encoder only for the Linux arm64 GNU build used by Remotion Lambda.
• Keep AV1 decode support and keep AV1 encoding enabled on the other desktop and server targets.
• Update the Docker-based Lambda smoke test to expect the AV1 encoder to be unavailable while preserving the regular AV1 assertion on the other platforms.

Test plan

• Run node test-ffmpeg.mjs locally on macOS and confirm the non-Lambda AV1 encoder path still passes.
• Let CI verify the Linux arm64 GNU Docker build skips the AV1 encoder while the other targets keep it.

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