PREFIX in /usr/bin/opengamepadui can sometimes be empty

[philmmanjaro]

In some cases the PREFIX value is empty:

SCRIPT_DIR=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" &>/dev/null && pwd)
PREFIX=$(dirname -- "${SCRIPT_DIR}")

Hardcoding it to /usr solved the issue for me when adding it to a new OS.

[pastaq]

ShadowApex is on vacation so you'll likely need to wait for him to get a detailed explanation, but we can't hardcode that to /usr due to the way NixOS handles directories IIRC. It might be okay to add some validation/fallback here though.

This may be related to #423