tests: point the audio probe at libobs' bundled ffmpeg

The new regression test measures recorded audio with ffmpeg's volumedetect,
but ffmpeg is not on the Windows runner, so it failed with ENOENT on all
three attempts.

libobs already ships ffmpeg.exe, and obs-studio-server/CMakeLists.txt
installs all of bin/64bit next to the osn binaries, so FFMPEG_PATH can point
at the copy that is already there. No install step and no new dependency.

Verified the bundled build (n7.1.1) has the volumedetect filter and reports
mean_volume: -34.1 dB for the test's own bigbuckbunny.mp4, well above the
-80 threshold the test asserts.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: summeroff

.github/workflows/main.yml

@@ -356,6 +356,9 @@ jobs:
           OSN_ACCESS_KEY_ID: ${{secrets.AWS_RELEASE_ACCESS_KEY_ID}}
           OSN_SECRET_ACCESS_KEY: ${{secrets.AWS_RELEASE_SECRET_ACCESS_KEY}}
           RELEASE_NAME: release
+          # ffmpeg is not on the runner, but libobs ships one and CMake installs all of
+          # bin/64bit next to the osn binaries, so point the audio probe at that copy
+          FFMPEG_PATH: ${{github.workspace}}\${{env.SLFullDistributePath}}\${{env.InstallPath}}\ffmpeg.exe
       # Run even after test failures so the PR still gets the flaky summary.
       - name: Publish flaky test check
         if: ${{ always() }}