Problem enabling transient /etc in fedora-based bootc image: rpm-ostree-fix-shadow-mode.service failed

Hello, while deploying an instance using the standard fedora:42 bootc image with transient etc enabled (an the initramfs re-generated) two systemd units relating to bootc and its ecosystem fail (in order):

1. rpm-ostree-fix-shadow-mode.service
2. bootc-fetch-apply-updates.service

After investigating a bit on my end, the only error message I was able to extract was the following:

```
[root@fedora]# rpm-ostree fix-shadow-perms
error: Remounting /boot read-write: Invalid argument
```

The same error appears when executing bootc upgrade --apply:

```
[root@fedora] # bootc upgrade --apply
error: Upgrading: Initializing storage: Acquiring sysroot: Remounting /boot read-write: Invalid argument
```

From what I gathered it looks more like an argument parsing issue than an actual /boot mount error. Digging into the rpm-ostree source code, the only place I think the error may be coming from is in the "fix_shadow_perms_entrypoint" function of the passwd.rs file. I stopped looking there, as that function calls some low level C/C++ bindings.

Additional information:

Fedora version: 42
Bootc version: 1.8.0
ostree/rpm-ostree versions: 2025.6

Would it be possible for your team to investigate on your end? /etc is crucial for us, and it is strongly recommended in your documentation.

Thanks in advance!

Inizia riproducendo il fallimento con `rpm-ostree fix-shadow-perms` e `bootc upgrade --apply` sull’immagine bootc Fedora 42 con /etc transitorio abilitato. Poi esamina `passwd.rs`, in particolare `fix_shadow_perms_entrypoint`, e i binding di basso livello che richiama. Il lavoro è completato quando entrambi i comandi terminano correttamente e le due unità systemd non falliscono più.

Scritto dal modello di indicizzazione a partire dal testo della issue.