flatpak: propagate glib and ostree

Pantheon's sideload broke:
```
meson.build:17:0: ERROR: Could not generate cargs for flatpak:
Package ostree-1 was not found in the pkg-config search path.
Perhaps you should add the directory containing `ostree-1.pc'
to the PKG_CONFIG_PATH environment variable
Package 'ostree-1', required by 'flatpak', not found
```

https://hydra.nixos.org/build/113077888

ZHF: #80379