CI: Update Flatpak's x264

In the future, a full FFMPEG build with x264 and nvenc will be
provided as an extension to the org.freedesktop.Platform runtime,
but for now we have to manually build it.

Update x264 to the latest git commit.

CI/flatpak/com.obsproject.Studio.json

@@ -38,7 +38,7 @@
         {
           "type": "git",
           "url": "https://code.videolan.org/videolan/x264.git",
-          "commit": "b86ae3c66f51ac9eab5ab7ad09a9d62e67961b8a"
+          "commit": "66a5bc1bd1563d8227d5d18440b525a09bcf17ca"
         }
       ]
     },