_v2-entry-point: pass every environment variable
Instead of manually whitelisting all the environment variables that we might need, we pass them all.
Pressure-vessel then will filter out the variables that are know to be wrong in the sandbox and the ones that will be edited.
This requires https://gitlab.steamos.cloud/steam/steam-runtime-tools/-/merge_requests/134
/cc @smcv
Edited by Simon McVittie