Address minor file descriptor leak in CollectTar.

Before this commit, the reader opened by `GetBlob` was not closed.
This does not matter for the current implementation of the S3 backend,
but for the FS backend the file descriptor was only closed by
the `os.file` finalizer.

V12-Ref: F-77205

src/collect.go

@@ -72,6 +72,9 @@ func CollectTar(
 				return
 			}
 			blobData, err = io.ReadAll(blobReader)
+			if closer, ok := blobReader.(io.Closer); ok {
+				defer closer.Close()
+			}
 			if err != nil {
 				return
 			}