fix: warn when $LESSCLOSE fails, not when it succeeds

The Drop cleanup for Preprocessed inverted the exit-status check, so bat
emitted a warning when LESSCLOSE exited successfully and stayed silent on
failure. Align the condition with the intended behavior.

src/lessopen.rs

@@ -261,7 +261,7 @@ impl Drop for Preprocessed {
                 }
             };
 
-            if lessclose_output.status.success() {
+            if !lessclose_output.status.success() {
                 bat_warning!("$LESSCLOSE exited with nonzero exit code",)
             };
         }