add very naive logic for launching in steam runtime

[Jan200101]

the logic added here is very naive and just copies the SDK 2013 mp lookup. it then runs $RUNTIME_DIR/_v2-entry-point -- hl2.sh to ensure everything is setup right

_v2-entry-point is labeled as a prototype so this might break in the future

$RUNTIME_DIR/run -- $RUNTIME_DIR/pressure-vessel/bin/steam-runtime-launcher-interface-0 container-runtime ./hl2.sh might be better? unsure

there are better ways of handling this in the code, this is mostly just to get things working without changing everything

[Jan200101]

its worth mentioning that there is a comment in hl2.sh that notes the dependency on the sniper runtime however there is no check to enforce this

# determine platform
UNAME=$(uname)
if [ "$UNAME" == "Linux" ]; then
    # TF2 requires the sniper container runtime
    . /etc/os-release

    # prepend our lib path to LD_LIBRARY_PATH
    export LD_LIBRARY_PATH="${GAMEROOT}/bin/$plat_dir":$LD_LIBRARY_PATH
fi

[wgetJane]

imo why not just a bash file in the game output folder?

[Jan200101]

the runtime can be installed anywhere so making use of the steam api to get it, like how its done for the actual engine build, is a lot more reliable