refactor: use io.Pipe and ffmpeg's stdin (#1148)

This removes the usage of `syscall.Mkfifo` which was previously used and
won't work on Windows systems and opens the door for other processes on
the computer to interfere in the rtmp stream (dumping bad content in the
fifo, removing the file, blocking the file in offline status).
Instead, this patch introduces an `io.Pipe` which pipes the RTMP stream
to the ffmpeg command while staying in Owncast.

Further links:
* ffmpeg on using `pipe:0` as an input: https://ffmpeg.org/ffmpeg-protocols.html#pipe

utils/utils.go

@@ -20,11 +20,6 @@ import (
 	"mvdan.cc/xurls"
 )
 
-// GetTemporaryPipePath gets the temporary path for the streampipe.flv file.
-func GetTemporaryPipePath(identifier string) string {
-	return filepath.Join(os.TempDir(), "streampipe."+identifier)
-}
-
 // DoesFileExists checks if the file exists.
 func DoesFileExists(name string) bool {
 	if _, err := os.Stat(name); err != nil {