ffmpeg: Checkout tag n4.3.1

Target a specific release version rather than some random development
commit.

externals/ffmpeg

@@ -1 +1 @@
-Subproject commit 81c462ad95f143837a6a21126117515577f0977b
+Subproject commit 6b6b9e593dd4d3aaf75f48d40a13ef03bdef9fdb