nixos-26.05 asserts against boot.initrd.postDeviceCommands under the
systemd stage-1 initrd. Move the root-subvolume rollback into an initrd
systemd oneshot (ordered after the labelled device, before sysroot.mount).
Same rollback logic; awk swapped for cut since coreutils' cut is in the
systemd initrd but awk is not.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>