Enhance build times disabling command-not-found during rootfs creation

[alexl83]

Description

fix issue #6531 by disabling "command-not-found" apt hook during build-time (which takes ages under qemu) and re-enabling after succesfully creating rootfs
No impact for end user as command-not-found will create/update its internal db natively/transparently on SBC during first sudo apt update

This shaves and additional ~2 minutes on a cached build with ORAS and CCACHE

How Has This Been Tested?

• Built vanilla trixie & bookworm & noble & jammy for opi03
• Built vanilla trixie & bookworm & noble & jammy for opi5-plus

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code

[rpardini]

I agree, c-n-f is useful yes, but adds a lot of overhead.
It's also unclear to me when the c-n-f database is updated: is it 1) when the first apt update happens after cnf is installed, 2) when c-n-f itself is installed or 3) somehow else?
This somehow relates to the synaptic index debacle we had a few years ago, which related to shipping-or-not the apt package lists in the images. Having those in increases the image size, but possibly lowers the time needed for the first apt update when running on the board.
I'd sincerely prefer to have everything in place (possibly at the very latest possible moment, so that it does not run/affect the build process), but also not ship c-n-f/synaptic/apt-lists in the image: leave it to the user doing the first apt-update after booting on real hardware. But not everyone agrees...

[alexl83]

I agree, c-n-f is useful yes, but adds a lot of overhead. It's also unclear to me when the c-n-f database is updated: is it 1) when the first apt update happens after cnf is installed, 2) when c-n-f itself is installed or 3) somehow else? This somehow relates to the synaptic index debacle we had a few years ago, which related to shipping-or-not the apt package lists in the images. Having those in increases the image size, but possibly lowers the time needed for the first apt update when running on the board. I'd sincerely prefer to have everything in place (possibly at the very latest possible moment, so that it does not run/affect the build process), but also not ship c-n-f/synaptic/apt-lists in the image: leave it to the user doing the first apt-update after booting on real hardware. But not everyone agrees...

I think it makes sense to have it installed in rootfs

cnf works like this:
/etc/apt/apt.conf.d/50command-not-found updates or re-creates database /var/lib/command-not-found/commands.db{,.metadata} everytime apt update is run (incrementally if db is already present)

stand-alone: root user can always run /usr/sbin/update-command-not-found to create/populate/update db at any given time (it was standard behaviour for first run after install many ubuntus ago) - it's somehow rendered useless by apt hook IMHO

While debootstrap and rootfs creation is non-interactive, cnf is basically useless at build-time: saving less than 20 seconds on native builds might be overkill and not useful for end-user, while saving 25-30 minutes on quemu builds makes all the sense of the world to me, especially since at first apt-update on SBC, cnf (which my changes re-enable before creating final image) will kick in natively and create its first database in less than 20 seconds during apt update

Everything is reasonably transparent to end user in my opinion, ponderating calculated impacts reported above
As an end-user I always appreciated cnf as it saved me countless apt queries to find which package to install in order to get specific binaries

regarding this:

I agree, c-n-f is useful yes, but adds a lot of overhead. It's also unclear to me when the c-n-f database is updated: is it 1) when the first apt update happens after cnf is installed, 2) when c-n-f itself is installed or 3) somehow else?

Correct answer is 1, with a twist - first apt update + every subsequent apt update (which will be incremental since first run already created a db) -- still during build time every subesquent run make build times a lot longer and excruciating by adding more pain

hope it helps :)