mantle/ore: gcloud: remove bucket subpath from GCE image name

Not sure why we needed this in the past but if I was trying to upload to
gs://mybucket/fedora-coreos my image name would get rewritten to append
the path `fedora-coreos` into the image name. For example:

- `fedora-coreos-31-20200404-dev-1-gcp-x86-64`

turns into

- `fedora-coreos-fedora-coreos-31-20200404-dev-1-gcp-x86-64`

This doesn't seem to be needed. Let's rework it so that transformation
doesn't happen.

mantle/cmd/ore/gcloud/upload.go

@@ -82,10 +82,10 @@ func runUpload(cmd *cobra.Command, args []string) {
 	}
 
 	uploadBucket = gsURL.Host
-	uploadImageName = strings.TrimPrefix(gsURL.Path+"/"+uploadImageName, "/")
-	// create equivalent image names for GS and GCE
+	imageNameGS := strings.TrimPrefix(gsURL.Path+"/"+uploadImageName, "/") + ".tar.gz"
+
+	// Sanitize the image name for GCE
 	imageNameGCE := gceSanitize(uploadImageName)
-	imageNameGS := uploadImageName + ".tar.gz"
 
 	storageAPI, err := storage.New(api.Client())
 	if err != nil {