image: always set filename for bootc raw pipeline

In BIB, we intend to export multiple images from the same pipeline and
this is the expected filename for the raw images.  Set it so that it's
always disk.raw even when we're building a qcow2 or other image type.

pkg/image/bootc_disk.go

@@ -49,6 +49,11 @@ func (img *BootcDiskImage) InstantiateManifestFromContainers(m *manifest.Manifes
 
 	opts := &baseRawOstreeImageOpts{useBootupd: true}
 	baseImage := baseRawOstreeImage(img.OSTreeDiskImage, buildPipeline, opts)
+
+	// In BIB, we intend to export multiple images from the same pipeline and
+	// this is the expected filename for the raw images. Set it so that it's
+	// always disk.raw even when we're building a qcow2 or other image type.
+	baseImage.SetFilename("disk.raw")
 	switch imgFormat {
 	case platform.FORMAT_QCOW2:
 		// qcow2 runs without a build pipeline directly from "bib"