heroic: add extraEnv input

Environment variables to pass to Heroic.

For example:

    pkgs.heroic.override {
      extraEnv.PROTON_ENABLE_WAYLAND = true;
    }

Adds feature parity with `steam` package.

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

@@ -3,6 +3,7 @@
   heroic-unwrapped,
   extraPkgs ? pkgs: [ ],
   extraLibraries ? pkgs: [ ],
+  extraEnv ? { },
 }:
 
 steam.buildRuntimeEnv {
@@ -12,7 +13,7 @@ steam.buildRuntimeEnv {
   runScript = "heroic";
 
   extraPkgs = pkgs: [ heroic-unwrapped ] ++ extraPkgs pkgs;
-  inherit extraLibraries;
+  inherit extraLibraries extraEnv;
 
   extraInstallCommands = ''
     mkdir -p $out/share