set content-type for manifests.txt

By default S3 treats uploaded files without a content-type as binary
blobs, which results in the browser downloading it when visiting the
URL. Setting the content-type to text/plain lets the browser render it.

Author: pietroalbini

src/main.rs

@@ -744,6 +744,7 @@ impl Context {
             .arg(manifest.path())
             .args(["--bucket", &self.config.upload_bucket])
             .args(["--key", "manifests.txt"])
+            .args(["--content-type", "text/plain"])
             // Fail the request if the manifest was already modified by something else (for
             // example, another release running in parallel).
             .args([conditional_header, &conditional_value]))?;