Flush streaming downloads from "cargo xtask download" (#8719)

This is another case like
#8681

(See:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=a9fdfeb0149e8ca0f84f3e164602da4e
for an example)

We need to flush after writing this data to ensure it actually leaves
our process

Author: smklein

dev-tools/downloader/src/lib.rs

@@ -275,6 +275,7 @@ async fn streaming_download(url: &str, path: &Utf8Path) -> Result<()> {
     while let Some(chunk) = response.chunk().await? {
         tarball.write_all(chunk.as_ref()).await?;
     }
+    tarball.flush().await?;
     Ok(())
 }