manifest: switch to halt --eject to avoid auto-reboot

In issue osbuild#230 it was raised that it is undesirable to automatically
reboot when the install is finished. So instead halt the install
and show a message to the user.

Closes: osbuild/bootc-image-builder#230

pkg/manifest/anaconda_installer_iso_tree.go

@@ -494,7 +494,7 @@ reqpart --add-boot
 part swap --fstype=swap --size=1024
 part / --fstype=ext4 --grow
 
-reboot --eject
+halt --eject
 `
 
 	// Workaround for lack of --target-imgref in Anaconda, xref https://github.com/osbuild/images/issues/380