meta path missing for newly built images

For the latest prgenv-gnu/25.06 images built from https://github.com/eth-cscs/alps-uenv/pull/224 we have
```
$ uenv image inspect --format='{meta}' prgenv-gnu/25.06:rc3
none
```
which is wrong. I assume this is caused by some change in #215, but have not investigated. This should probably be fixed before stackinator v2 recipes are considered ready.