koji: use nvra as the filename for images

We have the same thing for AWS. The AWS target also specifies under what name
should be the image available in EC2.

As requested by Brew maintainers Tomáš Kopeček and Lubomír Sedlář.

internal/target/koji_target.go

@@ -10,6 +10,7 @@ type KojiTargetOptions struct {
 	Filename        string `json:"filename"`
 	UploadDirectory string `json:"upload_directory"`
 	Server          string `json:"server"`
+	KojiFilename    string `json:"koji_filename"`
 }
 
 func (KojiTargetOptions) isTargetOptions() {}