New issue by mayhair on void-packages repository

https://github.com/void-linux/void-packages/issues/48876

Description:
### Is this a new report?

Yes

### System Info

Void 6.6.17_1 x86_64 GenuineIntel uptodate FFFFFFFFF

### Package(s) Affected

gdm-44.1_2

### Does a report exist for this bug with the project's home (upstream) and/or another distro?

_No response_

### Expected behaviour

It is possible to change the screen brightness via GDM, either by using the brightness keys or the graphical slider.

### Actual behaviour

Pressing the brightness keys or using the slider does nothing, graphically. It does output an error similar to this to socklog:
```
2024-02-22T09:17:48.62989 authpriv.crit: Feb 22 14:17:48 pkexec[2028]: gdm: The value for the SHELL variable was not found in the /etc/shells file [USER=root] [TTY=unknown] [CWD=/var/lib/gdm] [COMMAND=/usr/libexec/gsd-backlight-helper /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight 80]
```
This is peculiar, because executing the same command (`/usr/libexec/gsd-backlight-helper /sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-eDP-1/intel_backlight [brightness]`) within an interactive shell works with no problems.

### Steps to reproduce

When GDM is set as the display manager, make sure you are logged out.
Then try to change the brightness, either by pressing the brightness keys or by clicking the top-right corner and adjusting the brightness slider.