steam-fhsenv: allow building without Steam

I promise this will make sense.

pkgs/games/steam/fhsenv.nix

@@ -62,7 +62,7 @@ in buildFHSEnv rec {
   name = "steam";
 
   targetPkgs = pkgs: with pkgs; [
-    steamPackages.steam
+    steam
     # License agreement
     gnome.zenity
   ] ++ commonTargetPkgs pkgs;
@@ -207,10 +207,10 @@ in buildFHSEnv rec {
     libpsl
     nghttp2.lib
     rtmpdump
-  ] ++ steamPackages.steam-runtime-wrapped.overridePkgs
+  ] ++ steam-runtime-wrapped.overridePkgs
   ++ extraLibraries pkgs;
 
-  extraInstallCommands = ''
+  extraInstallCommands = lib.optionalString (steam != null) ''
     mkdir -p $out/share/applications
     ln -s ${steam}/share/icons $out/share
     ln -s ${steam}/share/pixmaps $out/share
@@ -262,9 +262,15 @@ in buildFHSEnv rec {
     exec steam ${extraArgs} "$@"
   '';
 
-  meta = steam.meta // lib.optionalAttrs (!withGameSpecificLibraries) {
-    description = steam.meta.description + " (without game specific libraries)";
-  };
+  meta =
+    if steam != null
+    then
+      steam.meta // lib.optionalAttrs (!withGameSpecificLibraries) {
+        description = steam.meta.description + " (without game specific libraries)";
+      }
+    else {
+      description = "Steam dependencies (dummy package, do not use)";
+    };
 
   # allows for some gui applications to share IPC
   # this fixes certain issues where they don't render correctly
@@ -298,7 +304,7 @@ in buildFHSEnv rec {
       exec -- "$run" "$@"
     '';
 
-    meta = steam.meta // {
+    meta = (steam.meta or {}) // {
       description = "Run commands in the same FHS environment that is used for Steam";
       name = "steam-run";
     };