Allow retrying HTTP/2 uploads for release assets

The `Request.GetBody` func allows the retry mechanism to reopen the file
that's being uploaded as the request body in case the body of the
previous request has already started to be read.

Hopefully fixes the error:

    http2: Transport: cannot retry err [stream error: stream ID 1; REFUSED_STREAM]
    after Request.Body was written; define Request.GetBody to avoid this error

Ref. d523dce5a7/http2/transport.go (L554)

pkg/cmd/release/shared/upload.go

@@ -170,6 +170,7 @@ func uploadAsset(httpClient *http.Client, uploadURL string, asset AssetForUpload
 	}
 	req.ContentLength = asset.Size
 	req.Header.Set("Content-Type", asset.MIMEType)
+	req.GetBody = asset.Open
 
 	resp, err := httpClient.Do(req)
 	if err != nil {