[downloader/external] Add another env for proxies in ffmpeg/avconv

Related sources:
https://git.libav.org/?p=libav.git;a=blob;f=libavformat/http.c;h=8fe8d11e1edfdbb04a8726db2c49cfef3f572aac;hb=HEAD#l152
https://git.libav.org/?p=libav.git;a=blob;f=libavformat/tls.c;h=fab243e93e20034e88e619188c13a44a5d8ccdb9;hb=HEAD#l63
https://github.com/FFmpeg/FFmpeg/blob/f8e89d8/libavformat/http.c#L191
https://github.com/FFmpeg/FFmpeg/blob/f8e89d8/libavformat/tls.c#L92

youtube_dl/downloader/external.py

@@ -210,6 +210,7 @@ class FFmpegFD(ExternalFD):
             # args += ['-http_proxy', proxy]
             env = os.environ.copy()
             compat_setenv('HTTP_PROXY', proxy, env=env)
+            compat_setenv('http_proxy', proxy, env=env)
 
         protocol = info_dict.get('protocol')