factory: handle properly snapd_recovery_mode based on modeenv value

[xnox]

Currently main, core23.10, core22, core20 have incomplete or incorrect handling of the factory mode.

As per #213

@kubiko are you going to provide updated merge request for main first? or do you want someone else to take over?

The likely intention is to consistently do https://github.com/snapcore/core-initrd/pull/213/files in all active code branches