Fix warn later extra whitespace

fmt::warning is whitespace agnostic. It's up to the BuildLog to enforce vertical whitespace. This matches warn_later output with existing warning output:

```
    fn warning(mut self: Box<Self>, s: &str) -> Box<dyn StartedAnnounceLogger> {
        writeln_now(&mut self.io, fmt::warning(s.trim()));
        writeln_now(&mut self.io, "");

        self
    }
```

commons/src/output/build_log.rs

@@ -236,7 +236,10 @@ where
     }
 
     fn warn_later(self: Box<Self>, s: &str) -> Box<dyn SectionAnnounceLogger> {
-        match crate::output::warn_later::try_push(fmt::warning(s.trim())) {
+        let mut formatted = fmt::warning(s.trim());
+        formatted.push('\n');
+
+        match crate::output::warn_later::try_push(formatted) {
             Ok(_) => self,
             Err(error) => {
                 eprintln!("[Buildpack Warning]: Cannot use the delayed warning feature due to error: {error}");
@@ -273,7 +276,10 @@ where
     }
 
     fn warn_later(self: Box<Self>, s: &str) -> Box<dyn StartedAnnounceLogger> {
-        match crate::output::warn_later::try_push(fmt::warning(s.trim())) {
+        let mut formatted = fmt::warning(s.trim());
+        formatted.push('\n');
+
+        match crate::output::warn_later::try_push(formatted) {
             Ok(_) => self,
             Err(error) => {
                 eprintln!("[Buildpack Warning]: Cannot use the delayed warning feature due to error: {error}");

commons/src/output/warn_later.rs

@@ -265,10 +265,9 @@ where
     fn drop(&mut self) {
         if let Some(warnings) = take() {
             if !warnings.is_empty() {
-                writeln!(&mut self.io).expect("warn guard IO is writeable");
                 for warning in &warnings {
-                    writeln!(&mut self.io, "{warning}").expect("warn guard IO is writeable");
                     writeln!(&mut self.io).expect("warn guard IO is writeable");
+                    write!(&mut self.io, "{warning}").expect("warn guard IO is writeable");
                 }
             }
         }
@@ -336,4 +335,18 @@ mod test {
 
         assert_contains!(reader.read_lossy().unwrap(), message);
     }
+
+    #[test]
+    fn does_not_double_whitespace() {
+        let writer = ReadYourWrite::writer(Vec::new());
+        let reader = writer.reader();
+        let guard = WarnGuard::new(writer);
+
+        let message = "Caution: This test is hot\n";
+        try_push(message).unwrap();
+        drop(guard);
+
+        let expected = "\nCaution: This test is hot\n".to_string();
+        assert_eq!(expected, reader.read_lossy().unwrap());
+    }
 }