feat(handheld): add pacman hook to force steam handheld branch

Adds a pacman hook that corrects the Steam launch arguments every time the package is updated. Currently unsure if this takes effect upon first install of CachyOS Handheld, however. The reason for this hook is because by default, Steam will execute with `-steamdeck` in Game Mode, but it does not do this in Desktop mode. This causes a problem when switching between the two modes, leading to `compatdata` and shader caches being stored in the wrong place, and also I've had a couple cases where it has bricked the Steam client when offline. Hard-forcing this to launch with the flag in desktop will fix this issue and matches SteamOS' behavior more closely.