proton: Populate NVIDIA_WINE_DLL_DIR when available

[liam-middlebrook]

NVIDIA-software may check the environment variable NVIDIA_WINE_DLL_DIR for a path where DLLs provided by the NVIDIA Linux driver can be found. This needs to be set by proton script, as it is ran inside of the pressure-vessel environment, so the paths returned will be valid within the environment.

Tested with an application that reads NVIDIA_WINE_DLL_DIR from the environment and tries to load a known DLL from it. As expected manually setting the environment variable to the on-disk location did not work, as the path is not accurate within the pressure-vessel environment.