append to log file

When resuming, append to the log file instead of clearing its old contents

av1an/src/main.rs

@@ -899,7 +899,7 @@ pub fn run() -> anyhow::Result<()> {
   for arg in args {
     // Change log file
     let new_log_file = FileSpec::try_from(PathAbs::new(&arg.log_file)?)?;
-    let _ = &logger.reset_flw(&flexi_logger::writers::FileLogWriter::builder(new_log_file))?;
+    let _ = &logger.reset_flw(&flexi_logger::writers::FileLogWriter::builder(new_log_file).append())?;
 
     Av1anContext::new(arg)?.encode_file()?;
   }