Revert "Revert "runtime: Set STEAM_RUNTIME=/ if running in scout"
Setting STEAM_RUNTIME=/ caused configuration changes in libraries like Pango and GTK, which broke module loading when not in the LD_LIBRARY_PATH Steam Runtime. We updated the affected libraries' search paths so it is safe now to reimplement the STEAM_RUNTIME change.
This reverts commit dc71943a.
/cc @smcv
I tested this with the latest Scout snapshot 0.20200920.0 and I can confirm that Floating Point's GTK menu correctly loads and also Feral's "Life is Strange: Before the Storm" (554620) works correctly while without this "STEAM_RUNTIME=/" patch it crashed at startup.