bib: use the Linux runner for building disk images

Distro-specific runners try to set up a build environment to satisfy
certain expectations for stages (things they might expect from a live OS
environment).  One of the things the Fedora runner does is run
`systemd-tmpfiles --create`.  This causes a lot of warnings in our new
buildroot, like the following:

    /usr/lib/tmpfiles.d/rpm-ostree-autovar.conf:31: Failed to resolve user 'rpcuser': No such process

likely because we are running in an "ostree-based" tree that isn't fully
deployed (/etc/passwd only contains root).

The Linux runner is a fallback runner that doesn't perform any
preparations.  This should be adequate for the stages we use in bib.

bib/cmd/bootc-image-builder/image.go

@@ -133,7 +133,7 @@ func manifestForDiskImage(c *ManifestConfig, rng *rand.Rand) (*manifest.Manifest
 
 	mf := manifest.New()
 	mf.Distro = manifest.DISTRO_FEDORA
-	runner := &runner.Fedora{Version: 39}
+	runner := &runner.Linux{}
 	containerSources := []container.SourceSpec{
 		{
 			Source:    c.Imgref,