stream_lavf: set --network-timeout to 60 seconds by default

Until now, we've made FFmpeg use the default network timeout - which is
apparently infinite. I don't know if this was changed at some point,
although it seems likely, as I was sure there was a more useful default.

For most use cases, a smaller timeout is more useful (for example
recording something in the background), so force a timeout of 1 minute.

See: #5793