steam: add json-glib to fhsenv

[mweinelt]

Motivation for this change

Fixes the Paradox Launcher for Stellaris.

Fixes: #123447

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[jonringer]

I don't have stellaris installed right now, but the PR looks good, and steam builds fine.

[jbalme]

I can confirm it works for me, thanks.

[jonringer]

Ironically, CK3 started to show this as well :)