pressure-vessel-unruntime: Support PRESSURE_VESSEL_APP_LD_LIBRARY_PATH
This supports the new environment variable "PRESSURE_VESSEL_APP_LD_LIBRARY_PATH" that "_v2-entry-point" will set.
The new variable has the advantage to support the eventual custom "LD_LIBRARY_PATH" from game launch options.
The related MR is steamlinuxruntime!19 (merged).
I left the backward compatibility to ease the transaction to the new variable.
If instead we are sure to ship the new _v2-entry-point and an updated pressure-vessel, I can remove the old export LD_LIBRARY_PATH="$SYSTEM_LD_LIBRARY_PATH"
/cc @smcv