Justfile needs a shortcut for 12th gen Framework 13

[castrojo]

We should find a smart way of adding multigenerations to the just shortcuts. The 12th gen needs this done: https://github.com/FrameworkComputer/linux-docs/blob/main/Fedora38-12thGen.md

The kargs from the existing just framework-13 probably work fine, but we should mirror the official docs when we can.

File is here:

https://github.com/ublue-os/framework/blob/main/system_files/just/custom.just

[bobslept]

Maybe we could use just framework-13 for last gen, and use just framework-13-gen12 for the 12th gen, and so on? When new a gen comes out, the current move to just framework-13-gen13 or something?

Or maybe read gen information from /proc/cpuinfo and act on that?

If we can a few grep -m 1 "model name" /proc/cpuinfo from framework users that would be awesome. On the framework forums I found a 12th gen one.

model name      : 12th Gen Intel(R) Core(TM) i5-1240P
model name	: 12th Gen Intel(R) Core(TM) i7-1260P

Seems doable by the looks of those two!