logtail: gate "logtail started" behind TS_DEBUG_LOGTAIL envknob (#19891)

Gates the unnecessary "logtail started" message behind
the debug envknob TS_DEBUG_LOGTAIL. This is extra log spam that isn't
needed unless we are debugging.

Updates tailscale/corp#40908

Signed-off-by: James Scott <jim@tailscale.com>

logtail/logtail.go

@@ -138,7 +138,9 @@ func NewLogger(cfg Config, logf tslogger.Logf) *Logger {
 	logger.uploadCancel = cancel
 
 	go logger.uploading(ctx)
-	logger.Write([]byte("logtail started"))
+	if envknob.Bool("TS_DEBUG_LOGTAIL") {
+		logger.Write([]byte("logtail started"))
+	}
 	return logger
 }