Upgrade install-playos script to perform the installation using RAUC

[yfyf]

Reasons:

• It manually implements mounting, file copy and etc instead of re-using rauc install
• It does not verify integrity of installation data
• It performs extra steps like manually parsing/updating status.ini, which should be done by RAUC and can become out of sync if RAUC is updated. E.g. statusfile is already deprecated in RAUC.
• It hardcodes certain paths, like /boot/status.ini

In general, this introduces unnecessary path divergence in terms of update process vs install process.

[yfyf]

Briefly looked into this and it's a bit tricky:

• Officially RAUC considers initial installs / bootstrapping to be out of scope
• There is rauc write-slot which could be used to perform the system image copy without the rest of RAUC update mechanics. Would solve some of the issues above, but not all. Not clear if it requires RAUC service to be running, probably yes.
• We can make the /boot path in base/self-update.nix configurable and re-use it in the installer with bootPath = "/mnt/boot". This should be sufficient to have a functional rauc.service after we finish partitioning.
• If RAUC was running, it would probably be confused about which slot it is booted into during installation. Could maybe explicitly mark it as slot.recovery or slot.installer (see RAUC A/B + recovery example)