[bug#69971] Some more information

[Abhishek Cherath]

The reason the driver path stuff is particularly important is that I get
my env vars for LIBVA_DRIVERS_PATH from guix package --search paths as
follows

```bash
eval "$(guix package --search-paths \
-p $HOME/.config/guix/current \
-p $HOME/.guix-profile \
-p $HOME/.guix-extra-profiles/emacs/emacs \
-p $HOME/.guix-home/profile \
-p /run/current-system/profile)"
```

and this gives the following for LIBVA_DRIVERS_PATH:
```bash
export 
LIBVA_DRIVERS_PATH="/run/current-system/profile/lib/dri:/home/abhishek/.guix-profile/lib/dri"
```

This means that any sandboxed program with access to one of those won't
be able to use hardware acceleration. I only figured this out when I got
curious about why mpv could use hardware accel just fine but nyxt
couldn't. It's also a problem for firefox. Guess I should put in a bug
report there?