Added environment variable PKMNSIM_DATABASE_DIR
Added environment variable PKMNSIM_DATABASE_DIR

* NSIS creates and deletes environment variable
* get_database_path() uses this successfully, but only on Linux

codemonkey85/LibPKMN,codemonkey85/LibPKMN,codemonkey85/LibPKMN,codemonkey85/LibPKMN,codemonkey85/LibPKMN