steam: add extraEnv option

nixos/modules/programs/steam.nix

@@ -14,7 +14,12 @@ in {
       defaultText = literalExpression "pkgs.steam";
       example = literalExpression ''
         pkgs.steam-small.override {
-          extraLibraries = with pkgs; [
+          extraEnv = {
+            MANGOHUD = true;
+            OBS_VKCAPTURE = true;
+            RADV_TEX_ANISO = 16;
+          };
+          extraLibraries = p: with p; [
             atk
           ];
         }

pkgs/games/steam/fhsenv.nix

@@ -4,6 +4,7 @@
 , extraLibraries ? pkgs: [ ] # extra packages to add to multiPkgs
 , extraProfile ? "" # string to append to profile
 , extraArgs ? "" # arguments to always pass to steam
+, extraEnv ? { } # Environment variables to pass to Steam
 , withGameSpecificLibraries ? true # exclude game specific libraries
 }:
 
@@ -52,6 +53,8 @@ let
     fi
   '';
 
+  envScript = lib.toShellVars extraEnv;
+
 in buildFHSUserEnv rec {
   name = "steam";
 
@@ -248,6 +251,9 @@ in buildFHSUserEnv rec {
 
     ${exportLDPath}
     ${fixBootstrap}
+
+    set -o allexport # Export the following env vars
+    ${envScript}
     exec steam ${extraArgs} "$@"
   '';
 
@@ -281,6 +287,9 @@ in buildFHSUserEnv rec {
 
       ${exportLDPath}
       ${fixBootstrap}
+
+      set -o allexport # Export the following env vars
+      ${envScript}
       exec -- "$run" "$@"
     '';