ci: Test installing a specific minimum FFmpeg version on Windows

Author: ximion

.github/workflows/build.yml

@@ -81,7 +81,7 @@ jobs:
     - name: Setup FFmpeg
       run: |
         vcpkg install --triplet x86-windows yasm-tool
-        vcpkg install --triplet x64-windows ffmpeg[core,avcodec,avfilter,avformat,swresample,swscale,vpx,opus,openssl]
+        vcpkg install --triplet x64-windows ffmpeg[core,avcodec,avfilter,avformat,swresample,swscale,vpx,opus,openssl]@7.1
 
     - name: Determine PoMiDAQ revision number
       shell: bash