polymc: use LD_LIBRARY_PATH, not GAME_LIBRARY_PATH
polymc: use LD_LIBRARY_PATH, not GAME_LIBRARY_PATH

Since 1.4.0, PolyMC simply uses LD_LIBRARY_PATH, rather than a custom
GAME_LIBRARY_PATH variable.

This change was made in PolyMC/PolyMC#893, which also fixes the in-tree
Nix wrapper script to match. NixOS/nixpkgs#182621 updated our PolyMC to
1.4.0, but did not port the wrapper changes along.

The lack of LD_LIBRARY_PATH causes libpulse to be unavailable,
causing the sound issues observed in NixOS/nixpkgs#184189.

Change-Id: I4042f774c1fc804609dbaa3f4206ca0dd8ab20b3

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs