ostree: Switch to composefs enabled = yes

This is the opposite workaround for
https://github.com/ostreedev/ostree-rs-ext/issues/612

However, we still need to finally make `/opt` a directory
to complete this, which currently wants changes in rpm-ostree.
(Or, we could tweak our build process to inject as a container
 afterwards)

tier-0/ostree.yaml

@@ -8,7 +8,7 @@ postprocess:
     mkdir -p /usr/lib/ostree
     cat > /usr/lib/ostree/prepare-root.conf << EOF
     [composefs]
-    enabled = true
+    enabled = yes
     [sysroot]
     readonly = true
     EOF