misc: Update to FFmpeg@8.1

[ldm0]

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[Copilot]

Pull request overview

Updates the default FFmpeg branch used by the platform build helper scripts and expands CI coverage to include FFmpeg release/8.1.

Changes:

• Bump the default FFmpeg git branch from release/8.0 to release/8.1 in the Linux/macOS/Windows FFmpeg build scripts.
• Add release/8.1 to the GitHub Actions CI matrix and ensure it is part of the “should_test” nightly set and feature-flag mapping.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
utils/windows_ffmpeg.rs	Update default FFmpeg branch argument to release/8.1 for Windows build helper.
utils/mac_ffmpeg.rs	Update default FFmpeg branch argument to release/8.1 for macOS build helper.
utils/linux_ffmpeg.rs	Update default FFmpeg branch argument to release/8.1 for Linux build helper.
.github/workflows/ci.yml	Add release/8.1 to CI matrix and feature-flag mapping; extend nightly test coverage list.

Comments suppressed due to low confidence (1)

.github/workflows/ci.yml:138

• The exclude comment says “Only run valgrind with latest FFmpeg”, but valgrind isn’t excluded for release/7.1 or release/8.0 (and now release/8.1 is the latest). Either update the exclude list to keep valgrind limited to the single latest version, or adjust the comment so it reflects the actual matrix behavior.

        exclude:
          # Only run valgrind with latest FFmpeg and stable rust toolchain to reduce resource consumption.
          # valgrind starts to fail on nightly toolchain due to a bug in std: https://github.com/rust-lang/rust/issues/135608
          - valgrind: "valgrind"
            ffmpeg-version: "release/6.0"
          - valgrind: "valgrind"
            ffmpeg-version: "release/6.1"
          - valgrind: "valgrind"
            ffmpeg-version: "release/7.0"
          - valgrind: "valgrind"

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