contrib: windows: use a working ffmpeg url + version

Change-Id: I207558e802ab2ba894deea07abefff1f6f855971

contrib/src/ffmpeg/package.json

 {
     "name": "ffmpeg",
-    "version": "n6.0",
-    "url": "https://git.ffmpeg.org/gitweb/ffmpeg.git/snapshot/__VERSION__.tar.gz",
+    "version": "6.0.1",
+    "url": "https://ffmpeg.org/releases/ffmpeg-__VERSION__.tar.xz",
     "deps": [
         "vpx",
         "x264",