ci: Pin windows ffmpeg build to version 8.1.2

Author: PolyMeilex

.github/workflows/build.yml

@@ -53,7 +53,7 @@ jobs:
     runs-on: windows-latest
 
     env:
-      FFMPEG_DOWNLOAD_URL: https://www.gyan.dev/ffmpeg/builds/ffmpeg-release-full-shared.7z
+      FFMPEG_DOWNLOAD_URL: https://www.gyan.dev/ffmpeg/builds/packages/ffmpeg-8.1.2-full_build-shared.7z
 
     steps:
       - uses: actions/checkout@v6

.github/workflows/release.yml

@@ -56,7 +56,7 @@ jobs:
     runs-on: windows-latest
 
     env:
-      FFMPEG_DOWNLOAD_URL: https://www.gyan.dev/ffmpeg/builds/ffmpeg-release-full-shared.7z
+      FFMPEG_DOWNLOAD_URL: https://www.gyan.dev/ffmpeg/builds/packages/ffmpeg-8.1.2-full_build-shared.7z
 
     steps:
       - uses: actions/checkout@v6