repo: Disable uncompressed object cache on non-writable repos

While we did support disabling the uncompressed-objects-cache
per-repository:

1) We didn't actually respect that operation when doing
   CHECKOUT_MODE_USER on archive-z2 repositories
2) It'd be better to automatically detect we can't write to the
   repo and disable the uncompressed cache then.

src/libostree/ostree-repo-private.h

@@ -58,6 +58,7 @@ struct OstreeRepo {
   GPtrArray *cached_content_indexes;
 
   gboolean inited;
+  gboolean writable;
   gboolean in_transaction;
   gboolean disable_fsync;
   GHashTable *loose_object_devino_hash;