Updated package isn't used for system reconfigure..?

[Simen Endsjø]

I updated mesa from 21.3.8 to 22.2.2 to see if I could get better support for my
Intel Iris Xe GPU on an Alder Lake CPU.

My change is reflected when doing `guix build mesa`, but not `guix system
reconfigure` – although I used `system reconfigure` while developing/debugging
the patch, so it was picked up somewhere, somehow at least.

```
$ ./pre-inst-env guix build mesa
/gnu/store/p6ck4a731rpkp43im3cx8cbgbyvq0yn2-mesa-22.2.2-bin
/gnu/store/g7w4xqnnk0cpqhh8h1la3fy45mb5rh2y-mesa-22.2.2
```

Compiling the system seems to work, and I get a new working configuration:

`$ sudo -E ./pre-inst-env guix system reconfigure ~/my-system.scm`

But after booting into the new system, `mesa-21.3.8` is still used:

```
$ inxi -G
Graphics:
  Device-1: Intel driver: i915 v: kernel
  Device-2: Acer Integrated RGB Camera type: USB driver: uvcvideo
  Display: x11 server: X.Org v: 21.1.4 driver: X: loaded: modesetting
    unloaded: fbdev,vesa gpu: i915 resolution: 1920x1200~60Hz
  OpenGL: renderer: Mesa Intel Graphics (ADL GT2) v: 4.6 Mesa 21.3.8
```