iio-sensor-proxy: delays shutdown on systemd installs
### Describe your issue
#### What's the expected behaviour?
quick shutdown with systemd
#### What's the current behaviour?
Sometimes it takes a long time to shutdown. The screen is already black and it seems like it has shutdown, but then takes another minute or so. I didn't investigate which service it is, but according to multiple reports in #3754, this is caused by the iio-sensor-proxy service.
#### How to reproduce your issue?
### What device are you using?
e.g. oneplus-enchilada
### On what postmarketOS version did you encounter the issue?
* [x] edge (`master` branch)
* [x] `v25.06`
* [ ] `v24.12`
* [x] I confirm that the issue still is present after running `sudo apk upgrade -a`
### On what environment did you encounter the issue?
#### Environments
Should affect all.
### Which init system are you using?
* [ ] OpenRC
* [x] systemd
### How did you get postmarketOS image?
* [ ] from https://images.postmarketos.org
* [ ] I built it using pmbootstrap
* [ ] It was preinstalled on my device
### What's the build date of the image? (in yyyy-mm-dd format)
### Additional information
issue