heroic: disable privateTmp

Disable privateTmp in the Steam runtime env wrapper so that Heroic does
not run multiple instances.

Closes #432037

pkgs/by-name/he/heroic/package.nix

···
   extraLibraries ? pkgs: [ ],
 }:
 
-steam.buildRuntimeEnv {
+(steam.override { privateTmp = false; }).buildRuntimeEnv {
   pname = "heroic";
   inherit (heroic-unwrapped) version meta;