steam: fix crashing by adding libnm to FHS

Following issue NixOS#238101 latest Steam and Proton (8.0+) will frequently
crash with a generic "GLib-GObject-CRITICAL". This error message ended
up being been a red-herring, and the actual issue lies within the
dependency on libnm inside Steam.

Fix for now is to use our own system libnm directly for the FHS.

Solution originally found in this Arch Linux issue:
https://bugs.archlinux.org/task/79006

pkgs/games/steam/fhsenv.nix

@@ -81,6 +81,7 @@ in buildFHSEnv rec {
     libGL
     libva
     pipewire.lib
+    (lib.getLib networkmanager) # for libnm.so
 
     # steamwebhelper
     harfbuzz